docs: admin-guide: bring some order to the "everything else" section

docs: admin-guide: bring some order to the "everything else" section

The bulk of the admin guide had become a big pile of stuff haphazardly
tossed together, mostly in the catch-all "everything else" section.  Split
that section into a few broad categories and sort the documents into them
as appropriate.

No documents have been added or removed, they are just reordered.  Note
that many of these documents are severely obsolete and should be considered
for removal.

Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20241213182057.343527-4-corbet@lwn.net