docs: standardize directory index to --- with overline

docs: standardize directory index to --- with overline

Use a standard heading format for the index.rst file in a directory.
Using overlines makes it clear that individual documents can use e.g.
=== for chapter titles and --- for section titles, as suggested in the
Linux kernel guidelines[1].  They could do it anyway, because documents
included in a toctree are parsed separately and therefore are not tied
to the same conventions for headings.  However, keeping some consistency is
useful since sometimes files are included from multiple places.

[1] https://www.kernel.org/doc/html/latest/doc-guide/sphinx.html

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>