Stop parsing from overwriting Sphinx configuration

[flying-sheep]

Fixes #421, fixes scverse/scanpy#2829

This PR introduces a parse function that abstracts away the instantiation and use of a RSTParser instance.

It uses the sphinx.util.docutils:sphinx_domains context manager to make sure that the docutils directive and role registries are managed properly.

[gaborbernat]

Let's add a test for this case 👍

[gaborbernat]

[flying-sheep]

Thanks, that was quick!