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.