docs: rework the driver-api top-level page

Add subsections in an attempt to bring a bit order to this page; also sort
most subsections into alphabetical order.  With luck all this will help to
prevent merge conflicts on this page due to everybody adding entries at the
end.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/87plxtm2oo.fsf@meer.lwn.net