Documentation: front page: use recommended heading adornments

Convert the Documentation front page to use the heading adornments
that are documented in doc-guide/sphinx.rst for document title and
chapters. I.e., convert most section headings to chapters.

This leaves "Indices and tables" as a chapter entry at the same level
as the other chapters.

The only visual difference from before to after is that the "Indices
and tables" heading is smaller and has more vertical whitespace
preceding it (although that may depend on the web browser being used).

Fixes: 0c7b4366 ("docs: Rewrite the front page")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/20230215005726.27320-1-rdunlap@infradead.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>