Merge branch 'mauro-pdf' into docs-next

Mauro says:

This series address a series of errors during PDF generation from
media documentation.

The first patch fixes the late redefinition of a LaTeX command at the
Sphinx LaTeX style that causes build to break when some cross-references
are used.

The next two patches fix PDF output issues with subdev-formats.rst.

The next 3 patches fix image includes and their output for PDF.

It is aligned with Linus request of not having binary-generated images
from their SVG source codes.

I still intend to move the remaing PNG images to vectorial ones (SVG),
as image scale works better, but this will require some additional work.
When done, I'll submit as a separate patch series.

It should also be noticed that the last patch violates the output dir,
when make is used with "O=some_dir", as Sphinx doesn't accept
image files outside the source directory. We'll likely need some Sphinx
extension in order to fix it, but at least with this series (plus Jani Nikola's
PDF fix series), the PDF output should work fine again.

[jc: added a commit fixing up a "make cleandocs" warning]