Infra: Exclude .git directory from Sphinx processing

[hugovk]

For #2404 (comment).

Whichever of #2405 and #2415 we choose, we definitely don't want Sphinx to process the contents of the .git directory. Let's explicitly exclude it.

[CAM-Gerlach]

#2405 didn't previously fully address this case, but as it would cause a merge conflict accepting this, I added this line there.

#2415 already addresses this (and this will also cause a merge conflict there), as it only attempts to read rst files from directories beginning with pep- with four following characters (and with my requested enhancement, more precisly only pep-NNNN directories as specified in PEP 1/PEP 12. Therefore, .git is naturally excluded, as is everything else on the current exclude list save the PEP template.

As such, it seems this PR isn't needed if either of those are merged (and conflicts with them), so merging this is only necessary if we want a very short term stopgap.

[hugovk]

Thanks, let's close this then!