docs: load_config.py: avoid needing a conf.py just due to LaTeX docs

Right now, for every directory that we need to have LaTeX output,
a conf.py file is required.

That causes an extra overhead and it is actually a hack, as
the latex_documents line there are usually a copy of the ones
that are there already at the main conf.py.

So, instead, re-use the global latex_documents var, just
adjusting the path to be relative ones.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>