[BD-21] Generate feature toggle documentation

We introduce a new documentation target, where we use the featuretoggles
Sphinx extension from code-annotations to generate human-readable
documentation of feature toggles in edx-platform. The annotation report
is generated on-the-fly based on the standard feature toggle
configuration file in code-annotations.

In addition, we add new doc.in & doc.txt requirement files that will be
pip-installed by readthedocs to generate the documentation targets.