docs: Fix the docs build with Sphinx 6.0

Sphinx 6.0 removed the execfile_() function, which we use as part of the
configuration process. They *did* warn us... Just open-code the
functionality as is done in Sphinx itself.

Tested (using SPHINX_CONF, since this code is only executed with an
alternative config file) on various Sphinx versions from 2.5 through 6.0.

Reported-by: Martin Liška <mliska@suse.cz>
Cc: stable@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

docs: load_config.py: ensure subdirs end with "/"

The logic with seeks for a subdir passed via SPHINXDIRS is
incomplete: if one uses something like:

make SPHINXDIRS=arm pdfdocs

It will find both "arm" and "arm64" directories. Worse than
that, it will convert "arm64/index" to "4/index".

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

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>

doc-rst: generic way to build only sphinx sub-folders

Add a generic way to build only a reST sub-folder with or
without a individual *build-theme*.

* control *sub-folders* by environment SPHINXDIRS
* control *build-theme* by environment SPHINX_CONF

Folders with a conf.py file, matching $(srctree)/Documentation/*/conf.py
can be build and distributed *stand-alone*. E.g. to compile only the
html of 'media' and 'gpu' folder use::

make SPHINXDIRS="media gpu" htmldocs

To use an additional sphinx-build configuration (*build-theme*) set the
name of the configuration file to SPHINX_CONF. E.g. to compile only the
html of 'media' with the *nit-picking* build use::

make SPHINXDIRS=media SPHINX_CONF=conf_nitpick.py htmldocs

With this, the Documentation/conf.py is read first and updated with the
configuration values from the Documentation/media/conf_nitpick.py.

Signed-off-by: Markus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>