Tune HTML theme parameters

Borrow two changes from the Linux kernel alabaster theme configuration:

- Make the sidebar fixed, for easier navigation.
- Change the font family to serif, to make the HTML visually closer to the
  pdf, and arguably a bit easier to read.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>

Author: vstehle

source/conf.py

@@ -94,7 +94,10 @@
 # further.  For a list of options available for each theme, see the
 # documentation.
 #
-# html_theme_options = {}
+html_theme_options = {
+    "fixed_sidebar": "true",
+    "font_family": "serif",
+}
 
 # Add any paths that contain custom static files (such as style sheets) here,
 # relative to this directory. They are copied after the builtin static files,