Improve mumble installation guide

docs/conf.py

@@ -101,7 +101,10 @@ html_theme = 'sphinx_rtd_theme'
 # further.  For a list of options available for each theme, see the
 # documentation.
 #
-# html_theme_options = {}
+
+html_theme_options = {
+    'navigation_depth': 5,
+}
 
 # 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,