[Sphinx] Adjust for source_parsers deprecation in Sphinx 3.0

Update the Sphinx configuration for the removal of source_parsers in
Sphinx 3.0. The variable has been deprecated since version 1.8.

Version 1.8 deprecates and version 3.0 removes the source_parsers
configuration variable that was used by older recommonmark versions.

https://www.sphinx-doc.org/en/master/usage/markdown.html

Differential revision: https://reviews.llvm.org/D75284