rustdoc: run on plain Markdown files.

This theoretically gives rustdoc the ability to render our guides,
tutorial and manual (not in practice, since the files themselves need to
be adjusted slightly to use Sundown-compatible functionality).

Fixes #11392.
rustdoc: add table-of-contents recording & rendering, use it with plain
markdown files.

This means that

    # Foo
    ## Bar
    # Baz
    ### Qux
    ## Quz

Gets a TOC like

    1 Foo
       1.1 Bar
    2 Baz
       2.0.1 Qux
       2.1 Quz

This functionality is only used when rendering a single markdown file,
never on an individual module, although it could very feasibly be
extended to allow modules to opt-in to a table of contents (std::fmt
comes to mind).
rustdoc: hardcode each header as a link.

This avoids having to include JS in the guide/tutorial/manual pages just
to get the headers being links. The on-hover behaviour showing the
little section marker § is preserved, because that gives a useful hint
that the heading is a link.
rustdoc: adding some common feature gates when testing a markdown file.

The manual, tutorial and guides need the feature gates quite often,
unfortunately, so this is the low-cost path to migrating to use
rustdoc. This is only activated for pure-Markdown files.

Preferably this would be avoided: #12773
docs: adjust code blocks to pass with rustdoc.

The changes are basically just because rustdoc runs tests/rendering on
more snippets by default (i.e. everything without a `notrust` tag), and
not anything significant.
mk: rewrite the documentation handling.

This converts it to be very similar to crates.mk, with a single list of
the documentation items creating all the necessary bits and pieces.

Changes include:
- rustdoc is used to render HTML & test standalone docs
- documentation building now obeys NO_REBUILD=1
- testing standalone docs now obeys NO_REBUILD=1
- L10N is slightly less broken (in particular, it shares dependencies
  and code with the rest of the code)
- PDFs can be built for all documentation items, not just tutorial and
  manual
- removes the obsolete & unused extract-tests.py script
- adjust the CSS for standalone docs to use the rustdoc syntax
  highlighting
mk: rename `check-...-doc-<crate>` to `check-...-doc-crate-<crate>`.

E.g. this stops check-...-doc rules for `rustdoc.md` and `librustdoc`
from stamping on each other, so that they are correctly built and
tested. (Previously only the rustdoc crate was tested.)
tutorial: hack a code snippet to make it compile.

This is meant to be compiling a crate, but the crate_id attribute seems
to be upsetting it if the attribute is actually on the crate. I.e. this
makes this test compile by putting the crate_id attribute on a function
and so it's ignored. Such a hack. :(
docs: render rustdoc docs with rustdoc, hack around sundown code-fence
parsing limitations.

Sundown parses

    ```
    ~~~

as a valid codeblock (i.e. mismatching delimiters), which made using
rustdoc on its own documentation impossible (since it used nested
codeblocks to demonstrate how testable codesnippets worked).

This modifies those snippets so that they're delimited by indentation,
but this then means they're tested by `rustdoc --test` & rendered as
Rust code (because there's no way to add `notrust` to
indentation-delimited code blocks). A comment is added to stop the
compiler reading the text too closely, but this unfortunately has to be
visible in the final docs, since that's the text on which the
highlighting happens.
auto merge of #12747 : huonw/rust/rustdoc-markdown, r=alexcrichton

This gives Rustdoc the ability to render our guides, tutorial and manual and modifies the those documents (minor modifications) and makefiles to achieve this.

See commits for more details (especially the makefile rewrite).