Fix bibtex v2

Bibtex v2 no longer automatically finds the .bib files (or at least not for our repo). Need to explicitly state them in conf.py, as far as I could tell has no effect on previous versions.

https://sphinxcontrib-bibtex.readthedocs.io/en/latest/usage.html#configuration

src/docs/sphinx_docs/conf.py

@@ -100,6 +100,7 @@ html_favicon = os.path.join('..','website','images','PETSc_RGB-logo.png')
 
 # -- Options for LaTeX output --------------------------------------------
 
+bibtex_bibfiles = ['../tex/petsc.bib','../tex/petscapp.bib']
 latex_engine = 'xelatex'
 
 # Specify how to arrange the documents into LaTeX files.