[Sphinx] Support older recommonmark versions.

The "new way" of enabling recommonmark is only supported in recommonmark
0.5 and later. Use the deprecated approach with versions of Sphinx that
still support it.

If I understand correctly there's no way to use older versions of
recommonmark (<0.5) with newer versions of Sphinx (>3.0) because the old
approach got removed.

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