Docs don't seem to properly list C++ methods anymore

[adamnovak]

The libbdsg ReadTheDocs docs, separate from the vg Doxygen docs, have a Doxygen-derived listing of C++ methods for the handle graph API, at https://bdsg.readthedocs.io/en/master/rst/cppapi.html

Even in the last successful docs build, this seems to have been broken. The Doxygen output didn't make it to Sphinx properly to build the web page.

We should fix it so the stuff included over from Doxygen shows up on the page again.