docs/conf.py: Use about.html only in sidebar of alabaster theme

"about.html" is available only for the alabaster theme [1].
Unconditionally putting it to html_sidebars prevents us from
using other themes which respect html_sidebars.

Remove about.html from the initialization and insert it at the
front for the alabaster theme.

Link: [1] https://alabaster.readthedocs.io/en/latest/installation.html#sidebars
Fixes: d5389d31

 ("docs: Switch the default HTML theme to alabaster")
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/4b162dbe-2a7f-1710-93e0-754cf8680aae@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>