docs: tweak some Alabaster style parameters

This is just the beginning: tighten up the layout a bit to improve the
information density in the browser.  Also reconfigure the page width in
terms of character units (em) rather than pixels, making it more
display-independent.  To that end, add a custom.css file to
tweak Alabaster CSS settings.

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

Documentation/conf.py

@@ -345,6 +345,9 @@ if  html_theme == 'classic':
 else:
     html_theme_options = {
         'description': get_cline_version(),
+        'font_size': '10pt',
+        'page_width': '65em',
+        'sidebar_width': '15em',
     }
 
 sys.stderr.write("Using %s theme\n" % html_theme)