docs: install sphinx theme with python

Author: jankapunkt

.readthedocs.yml

@@ -17,6 +17,6 @@ sphinx:
 
 # We recommend specifying your dependencies to enable reproducible builds:
 # https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
-# python:
-#   install:
-#   - requirements: docs/requirements.txt
+python:
+  install:
+    - requirements: docs/requirements.txt

docs/requirements.txt

@@ -0,0 +1,4 @@
+# Defining the exact version will make sure things don't break
+sphinx==5.3.0
+sphinx_rtd_theme==1.1.1
+readthedocs-sphinx-search==0.1.1