Add RTD requirements

(cherry picked from commit 24186ef)

Author: dennissiemensma

.readthedocs.yaml

@@ -17,3 +17,7 @@ sphinx:
 # Build documentation with MkDocs
 #mkdocs:
 #  configuration: mkdocs.yml
+
+python:
+  install:
+    - requirements: docs/requirements.txt

docs/requirements.txt

@@ -0,0 +1,4 @@
+sphinx==4.3.2
+sphinx-rtd-theme==1.3.0
+sphinx-autobuild==2021.3.14
+sphinx-intl==2.1.0