Install distgen from git before building docs

This will ensure that we can grab version from distgen metadata
(pyproject.toml) during docs build

.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: docs/requirements.txt

docs/requirements.txt

@@ -0,0 +1 @@
+-e .