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.