docs: doc-guide: clarify latest theme usage

docs: doc-guide: clarify latest theme usage

We have been using the Alabaster theme as the default theme since
Sept. 2022.

Keep the information on the RTD theme around in case someone wants to
try it with the DOCS_THEME environment variable.

Fixes: 26d797ffc1c0 ("docs: update sphinx.rst to reflect the default theme change")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20250519223613.37277-1-rdunlap@infradead.org>