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.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/conf.py

@@ -345,7 +345,11 @@ html_use_smartypants = False
 
 # Custom sidebar templates, maps document names to template names.
 # Note that the RTD theme ignores this
-html_sidebars = { '**': ["about.html", 'searchbox.html', 'localtoc.html', 'sourcelink.html']}
+html_sidebars = { '**': ['searchbox.html', 'localtoc.html', 'sourcelink.html']}
+
+# about.html is available for alabaster theme. Add it at the front.
+if html_theme == 'alabaster':
+    html_sidebars['**'].insert(0, 'about.html')
 
 # Output file base name for HTML help builder.
 htmlhelp_basename = 'TheLinuxKerneldoc'