add bibtex_bibfiles to release so it builds

investigating incorrect patch release version on readthedocs

Commit-type: bug-fix Reported-by: Pierre Jolivet pierre@joliv.et Thanks-to: Jacob Faibussowitsch jacob.fai@gmail.com

See https://docs.petsc.org/en/barry-2020-12-26-docs-add-bibtext-release/ for what the results look like. Note it lists it as 3.14.2