docs: conf.py: fix support for Readthedocs v 1.0.0

docs: conf.py: fix support for Readthedocs v 1.0.0

As described at:
	https://stackoverflow.com/questions/23211695/modifying-content-width-of-the-sphinx-theme-read-the-docs

since Sphinx 1.8, the standard way to setup a custom theme is
to use html_css_files. While using html_context is OK with RTD
0.5.2, it doesn't work with 1.0.0, causing the theme to not load,
producing a very weird html.

Tested with:
	- Sphinx 1.7.9 + sphinx-rtd-theme 0.5.2
	- Sphinx 2.4.4 + sphinx-rtd-theme 0.5.2
	- Sphinx 2.4.4 + sphinx-rtd-theme 1.0.0
	- Sphinx 4.3.0 + sphinx-rtd-theme 1.0.0

Reported-by: Hans Verkuil <hverkuil@xs4all.nl>
Tested-by: Hans Verkuil <hverkuil@xs4all.nl>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Tested-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/r/80009f0d17ea0840d81e7e16fff6e7677919fdfc.1638004294.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>