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.