docs: update rtd to use sphinxcontrib-bibtex v2.0.0 and later

[andrew-platt]

This PR is ready to merge.

Summary
I had locked the sphinxcontrib-bibtex version to < 2.0.0 when I ran into issues with multiple bibliographies that we use (one per module). This was increasingly becoming a problem as python 3.8 and later would like to use a newer version.

Solution
The conf.py was updated to include a bibtex_bibfiles variable. However, the new version of sphinxcontrib-bibtex is more thorough in checking for repeated bibtex entries, and complained about a few duplicated entries (checked across all bib files). This required hardcoding of the keyprefix values into each modules .bib file instead of using sphinx to insert them. The rendering is therefore identical to the previous behavior, but will now compile correctly with the newer package.

See https://ap-openfast.readthedocs.io/en/f-docs-bibtex2.0.0/source/user/fast.farm/zrefs.html (duplicated entries first appeared in the fast.farm docs).

[andrew-platt]

@bjonkman, this should fix the python version issue you ran into with the documentation builds.

[bjonkman]

Thanks, @andrew-platt!