build: Add base requirements to docs.

If we want to build autodocs from the code, we need the dependencies of
the code installed as a part of the docs build.

requirements/edx/doc.in

@@ -1,6 +1,7 @@
 # Requirements for documentation validation
 -c ../constraints.txt
 
+-r base.txt
 code-annotations          # provides annotations for certain documentation
 sphinx-book-theme         # Common theme for all Open edX projects
 gitpython                 # fetch git repo information