docs: rework the userspace-api top page

Add some subsection headings and reorder entries so that the page makes a
bit more sense.  With luck, adding some ordering will also reduce merge
conflicts due to everybody adding new entries at the end.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/87ttn5m2q1.fsf@meer.lwn.net