docs: Rewrite the front page

The front page is the entry point to the documentation, especially for
people who read it online.  It's a big mess of everything we could think to
toss into it.  Rewrite the page with an eye toward simplicity and making it
easy for readers to get going toward what they really want to find.

This is only a beginning, but it makes our docs more approachable than
before.

Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: David Vernet <void@manifault.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20220927160559.97154-3-corbet@lwn.net
Signed-off-by: Jonathan Corbet <corbet@lwn.net>