docs: Switch the default HTML theme to alabaster

The read-the-docs theme is not entirely attractive and doesn't give us
control over the left column.  "Alabaster" is deemed the default Sphinx
theme, it is currently maintained and shipped bundled with Sphinx itself,
so there is no need to install it separately.  Switch over to this theme as
the default for building kernel documentation; the DOCS_THEME environment
variable can still be used to select a different theme.

Acked-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>