docs: add support to build manpages from kerneldoc output

docs: add support to build manpages from kerneldoc output

Generating man files currently requires running a separate
script. The target also doesn't appear at the docs Makefile.

Add support for mandocs at the Makefile, adding the build
logic inside sphinx-build-wrapper, updating documentation
and dropping the ancillary script.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Message-ID: <3d248d724e7f3154f6e3a227e5923d7360201de9.1758196090.git.mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>