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 <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/conf.py

···
 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)

Documentation/sphinx-static/custom.css

···
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * CSS tweaks for the Alabaster theme
+ */
+
+/* Shrink the headers a bit */
+div.body h1 { font-size: 180%; }
+div.body h2 { font-size: 150%; }
+div.body h3 { font-size: 130%; }
+
+/* Tighten up the layout slightly */
+div.body { padding: 0 15px 0 10px; }
+div.document { margin: 20px 10px 0 10px; }
+div.sphinxsidebarwrapper { padding: 1em 0.4em; }