docs: sphinx-requirements: Move sphinx_rtd_theme to top

docs: sphinx-requirements: Move sphinx_rtd_theme to top

sphinx_rtd_theme 0.5.2 has "docutils<0.17" in its requirements.
docutils 0.17 released this April caused regression in
sphinx_rtd_theme 0.5.1 [1].

By removing docutils and moving sphinx_rtd_theme before Sphinx in
requirements.txt, the requirement of "docutils<0.17" can be met
naturally.

[1]: https://github.com/readthedocs/sphinx_rtd_theme/issues/1112

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Acked-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/75f14c88-6091-1072-41cb-16b886aee5a0@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>