1.2.1: cannot build documentation without installing module

### Describe the bug

Looks like something is wrong and I cannot build docuemtation without installing module.


### Steps/Code to Reproduce

N/A

### Expected Results

It should be possible to build documentation out of only what is in source tree a d withoit have installed module.

### Actual Results

<details>

```console
++ ls -1d lib.linux-x86_64-cpython-38
+ PYTHONPATH=/home/tkloczko/rpmbuild/BUILD/scikit-learn-1.2.1/build/lib.linux-x86_64-cpython-38
+ PBR_VERSION=1.2.1
+ PDM_PEP517_SCM_VERSION=1.2.1
+ SETUPTOOLS_SCM_PRETEND_VERSION=1.2.1
+ /usr/bin/sphinx-build -n -T -b man doc build/sphinx/man
Running Sphinx v6.1.3

Traceback (most recent call last):
  File "/home/tkloczko/rpmbuild/BUILD/scikit-learn-1.2.1/sklearn/__check_build/__init__.py", line 48, in <module>
    from ._check_build import check_build  # noqa
ModuleNotFoundError: No module named 'sklearn.__check_build._check_build'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.8/site-packages/sphinx/config.py", line 351, in eval_config_file
    exec(code, namespace)  # NoQA: S102
  File "/home/tkloczko/rpmbuild/BUILD/scikit-learn-1.2.1/doc/conf.py", line 20, in <module>
    from sklearn.externals._packaging.version import parse
  File "/home/tkloczko/rpmbuild/BUILD/scikit-learn-1.2.1/sklearn/__init__.py", line 81, in <module>
    from . import __check_build  # noqa: F401
  File "/home/tkloczko/rpmbuild/BUILD/scikit-learn-1.2.1/sklearn/__check_build/__init__.py", line 50, in <module>
    raise_build_error(e)
  File "/home/tkloczko/rpmbuild/BUILD/scikit-learn-1.2.1/sklearn/__check_build/__init__.py", line 31, in raise_build_error
    raise ImportError(
ImportError: No module named 'sklearn.__check_build._check_build'
___________________________________________________________________________
Contents of /home/tkloczko/rpmbuild/BUILD/scikit-learn-1.2.1/sklearn/__check_build:
__init__.py               _check_build.pyx          _check_build.c
__pycache__
___________________________________________________________________________
It seems that scikit-learn has not been built correctly.

If you have installed scikit-learn from source, please do not forget
to build the package before using it: run `python setup.py install` or
`make` in the source directory.

If you have used an installer, please check that it is suited for your
Python version, your operating system and your platform.

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/lib/python3.8/site-packages/sphinx/cmd/build.py", line 279, in build_main
    app = Sphinx(args.sourcedir, args.confdir, args.outputdir,
  File "/usr/lib/python3.8/site-packages/sphinx/application.py", line 202, in __init__
    self.config = Config.read(self.confdir, confoverrides or {}, self.tags)
  File "/usr/lib/python3.8/site-packages/sphinx/config.py", line 173, in read
    namespace = eval_config_file(filename, tags)
  File "/usr/lib/python3.8/site-packages/sphinx/config.py", line 364, in eval_config_file
    raise ConfigError(msg % traceback.format_exc()) from exc
sphinx.errors.ConfigError: There is a programmable error in your configuration file:

Traceback (most recent call last):
  File "/home/tkloczko/rpmbuild/BUILD/scikit-learn-1.2.1/sklearn/__check_build/__init__.py", line 48, in <module>
    from ._check_build import check_build  # noqa
ModuleNotFoundError: No module named 'sklearn.__check_build._check_build'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.8/site-packages/sphinx/config.py", line 351, in eval_config_file
    exec(code, namespace)  # NoQA: S102
  File "/home/tkloczko/rpmbuild/BUILD/scikit-learn-1.2.1/doc/conf.py", line 20, in <module>
    from sklearn.externals._packaging.version import parse
  File "/home/tkloczko/rpmbuild/BUILD/scikit-learn-1.2.1/sklearn/__init__.py", line 81, in <module>
    from . import __check_build  # noqa: F401
  File "/home/tkloczko/rpmbuild/BUILD/scikit-learn-1.2.1/sklearn/__check_build/__init__.py", line 50, in <module>
    raise_build_error(e)
  File "/home/tkloczko/rpmbuild/BUILD/scikit-learn-1.2.1/sklearn/__check_build/__init__.py", line 31, in raise_build_error
    raise ImportError(
ImportError: No module named 'sklearn.__check_build._check_build'
___________________________________________________________________________
Contents of /home/tkloczko/rpmbuild/BUILD/scikit-learn-1.2.1/sklearn/__check_build:
__init__.py               _check_build.pyx          _check_build.c
__pycache__
___________________________________________________________________________
It seems that scikit-learn has not been built correctly.

If you have installed scikit-learn from source, please do not forget
to build the package before using it: run `python setup.py install` or
`make` in the source directory.

If you have used an installer, please check that it is suited for your
Python version, your operating system and your platform.


Configuration error:
There is a programmable error in your configuration file:

Traceback (most recent call last):
  File "/home/tkloczko/rpmbuild/BUILD/scikit-learn-1.2.1/sklearn/__check_build/__init__.py", line 48, in <module>
    from ._check_build import check_build  # noqa
ModuleNotFoundError: No module named 'sklearn.__check_build._check_build'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.8/site-packages/sphinx/config.py", line 351, in eval_config_file
    exec(code, namespace)  # NoQA: S102
  File "/home/tkloczko/rpmbuild/BUILD/scikit-learn-1.2.1/doc/conf.py", line 20, in <module>
    from sklearn.externals._packaging.version import parse
  File "/home/tkloczko/rpmbuild/BUILD/scikit-learn-1.2.1/sklearn/__init__.py", line 81, in <module>
    from . import __check_build  # noqa: F401
  File "/home/tkloczko/rpmbuild/BUILD/scikit-learn-1.2.1/sklearn/__check_build/__init__.py", line 50, in <module>
    raise_build_error(e)
  File "/home/tkloczko/rpmbuild/BUILD/scikit-learn-1.2.1/sklearn/__check_build/__init__.py", line 31, in raise_build_error
    raise ImportError(
ImportError: No module named 'sklearn.__check_build._check_build'
___________________________________________________________________________
Contents of /home/tkloczko/rpmbuild/BUILD/scikit-learn-1.2.1/sklearn/__check_build:
__init__.py               _check_build.pyx          _check_build.c
__pycache__
___________________________________________________________________________
It seems that scikit-learn has not been built correctly.

If you have installed scikit-learn from source, please do not forget
to build the package before using it: run `python setup.py install` or
`make` in the source directory.

If you have used an installer, please check that it is suited for your
Python version, your operating system and your platform.
```
</details>

### Versions

```shell
1.2.1
```

COMMENT:
> To build the documentation including the examples, scikit-learn will need to be installed anyways. However if you can find a simple way to build the doc (without the examples) and without installing scikit-learn then why not.

Issue is that during typical packaging process execution there is no chance to to have module installed ... because module is build from scratch. Biggest obstacle on packaging that way are relative imports because they are forces to have DSOs in the actual source tree.
I have at he moment +170KB patch which replaces all relative imports by module name(s) and that patch still is not finished.
With such patch it is possible to build documentation with two adjustments:
-  add in conf.py `sys.path.insert()` which injects path to the source tree. In case of your module I have it already done and it looks like
```patch
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -12,6 +12,8 @@

 import sys
 import os
+sys.path.insert(0, os.path.abspath(".."))
+
 import warnings
 import re
 from datetime import datetime
```
If you will look om sphinx example copy.py from https://www.sphinx-doc.org/en/master/usage/configuration.html#example-of-configuration-file you can find that as part of that example
- In case of modules which are using DSOs it is necessary to add second adjustment during `sphinx-build` command  execution which looks like
```console
[scikit-learn]$ PYTHONPATH=$PWD/build/$(cd build; ls -1d lib*) sphinx-build <params>
```
Those two adaptations allows open .py files from source tree and DSOs from second location where typical pep build procedure creates DSOs.

Benefit of above is that with those adaptations it is possible to build documentation without have installed it and by this have 100% guarantee that produced documentation will be generated not against what is installed (which may be different version) but what is in working source code tree.

Most of the python modules which are using DSOs already have above adaptation implemented.
Looks like `scikit-learn` is one of those rare cesses when that approach is not already implemented.

Labels: Bug, Documentation, Needs Decision - Include Feature