fix(readthedocs) add a requirement for the python tools needed by shpinx

Signed-off-by: Benoit Mortier <benoit.mortier@fusiondirectory.org>

.readthedocs.yaml

@@ -30,6 +30,6 @@ sphinx:
 # Optional but recommended, declare the Python requirements required
 # to build your documentation
 # See https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
-# python:
-#    install:
-#    - requirements: docs/requirements.txt
+ python:
+    install:
+    - requirements: source/requirements.txt

source/requirements.txt

+# Defining the exact version will make sure things don't break
+sphinx==3.4.3
+sphinx_rtd_theme==0.5.1