Increased the sidebar navigation TOC depth

Thanks to https://stackoverflow.com/questions/27669376/show-entire-toctree-in-read-the-docs-sidebar

docs/conf.py

@@ -48,3 +48,7 @@
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".
 html_static_path = []
+
+html_theme_options = {
+    'navigation_depth': 4,
+}