docs: Clean up and organize the admin guide a bit

The admin guide is a good start, but it's time to turn it into something
better than an unordered blob of files.  This is a first step in that
direction.  The TOC has been split up and annotated, the guides have been
reordered, and minor tweaks have been applied to a few of them.

One consequence of splitting up the TOC is that we don't really want to use
:numbered: anymore, since the count resets every time and there doesn't
seem to be a way to change that.  Eventually we probably want to group the
documents into sub-books, at which point we can go back to a single TOC,
but it's probably early to do that.

Reviewed-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>