doc: Makefile: if sphinx is not found, run a check script

Right now, if the building system doesn't find Sphinx, it
bails out, without providing any instructions about what
should be done.

Instead, run a script, providing some guidance about the
steps needed for Sphinx build to work.

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