docs: use sphinx_rtd_theme for HTML output

It makes sense to render using the same theme locally that is used on RTD.
The theme can be installed using:

    pip install sphinx_rtd_theme

docs/conf.py

@@ -74,7 +74,7 @@ pygments_style = None
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
 #
-html_theme = 'alabaster'
+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