ReadtheDocs Breathe for our API documentation

[laceysanderson]

In 2018, @spficklin and @bradfordcondon attempted to add breathe to our ReadtheDocs so that the API documentation could live alongside our other documentation and be easier to manage. At the time they came across a number of errors and thus it was not completed.

These were the original attempts at this:
rtd_breathe.patch.txt

I'm creating this issue in the hopes that we find time to re-visit this for our Tripal 4 documentation.