docs: conf.py: better handle latex documents

docs: conf.py: better handle latex documents

The original logic assumed that app.srcdir is identical to the
current working dir. This is the case for a normal build, but,
when SPHINXDIRS="some dir" is used, this is not the case anymore.

Adjust the logic to fill the LaTeX documents considering
app.srcdir, in a way that it will work properly on all cases.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/de23b35a770210950c609deaa32b98cb3673a53a.1755763127.git.mchehab+huawei@kernel.org