Use sphinx_rtd_theme also as an extension

As per their documentation: https://sphinx-rtd-theme.readthedocs.io/en/latest/installing.html#how-to-install-and-use-the-theme

docs/conf.py

@@ -9,6 +9,7 @@
     "sphinx.ext.autodoc",
     "sphinx.ext.intersphinx",
     "sphinx_autodoc_typehints",
+    "sphinx_rtd_theme",
 ]
 
 templates_path = ["_templates"]