Documentation: sort _SPHINXDIRS for 'make help'

Sort the _SPHINXDIRS so that the 'make help' output is easier to read &
search and in a predictable order instead of some unknown pseudo-random
order.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>