docs: add support for RTD dark mode

This is actually an overlay on the top of the RTD theme, which
requires to include first the RTD theme.

It should be noticed that, when the dark theme is used, the
DOCS_CSS files won't be the last CSS themes. So, it won't
override the dark.css style by default. So, it is needed
to force the them override with "!important".

This small script, for instance, produces a nice output with
the RTD dark theme:

DOCS_THEME=sphinx_rtd_dark_mode
cat << EOF > dark_override.css
html body {
font-family: arial,helvetica,sans-serif;
}
html[data-theme='dark'] body {
color: white !important;
}
html[data-theme='dark'] .sig-name {
color: green !important;
}
html[data-theme='dark'] .wy-menu-vertical a {
color: #ffcc00 !important;
}
html[data-theme="dark"] h1, html[data-theme="dark"] h2, html[data-theme="dark"] h3 {
color: #ffcc00 !important;
}
html[data-theme="dark"] h4, html[data-theme="dark"] h5, html[data-theme="dark"] h6 {
color: #ffcc00 !important;
}
html[data-theme="dark"] h7, html[data-theme="dark"] h8, html[data-theme="dark"] h9 {
color: #ffcc00 !important;
}
html[data-theme="dark"] .wy-nav-content a, html[data-theme="dark"] .wy-nav-content a:visited {
color: #ffcc00 !important;
}
EOF

make DOCS_CSS=dark_override.css DOCS_THEME=sphinx_rtd_dark_mode htmldocs

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/90d316e055ef7f4c9021b9eada8f8d3b2e750a66.1638870323.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>