docs: switch to Read the Docs theme

the "Read the Docs" Sphinx theme provides a sidebar
that is consistent across all pages,
which makes it easier to navigate.

Author: minijackson

docs/source/conf.py

@@ -91,7 +91,7 @@
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
-html_theme = 'default'
+html_theme = 'sphinx_rtd_theme'
 
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the