Investigate why local docs builds were causing errors due to site-packages

[willingc]

Follow-up to #628

Thanks @willingc – it makes sense, though for another time I wonder why we have venvs under the doc root in the first place 😅

Originally posted by @agriyakhetarpal in #628 (review)

[willingc]

Let's revisit our .gitignore settings.

[willingc]

Today I learned

.gitignore is fine for Git, but it does not control what Sphinx scans.

Why Sphinx is crawling .venv:

In noxfile.py, Sphinx is run with SOURCE_DIR = "." (repo root).
So Sphinx recursively discovers docs from the whole repo.
Sphinx only skips paths in conf.py exclude_patterns, not .gitignore.
Your current exclude_patterns includes .nox but does not include .venv.
So .venv gets treated as documentation content, and Sphinx/Myst starts parsing files in site-packages.

[willingc]

Closed by #628