docs: allow selecting a Sphinx theme

Instead of having RTD as an almost mandatory theme, allow the
user to select other themes via DOCS_THEME environment var.

There's a catch, though: as the current theme override logic is
dependent of the RTD theme, we need to move the code which
adds the CSS overrides to be inside the RTD theme logic.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/bd20adabfd428fd3cd0e69c2cf146aa354932936.1638870323.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

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