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: d5389d3145ef ("docs: Switch the default HTML theme to alabaster")
Signed-off-by: Akira Yokosawa <[email protected]>
Cc: Mauro Carvalho Chehab <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jonathan Corbet <[email protected]>

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