MAINT: Fixes for sphinx 7.3

[larsoner]

Fix for latest Sphinx leveraging sphinx-gallery/sphinx-gallery#1289

The solution is just to move a bunch of code to sphinxext/mne_doc_utils.py and then pass "mne_doc_utils.<whatever>" as str config values to sphinx_gallery_conf

[larsoner]

@drammock should be ready for manual merge since I skipped azure and actions

[drammock]

we should also really update line 82 of conf.py (needs_sphinx = "2.0") to match pyproject.toml (which says >=6). It's too far outside the diff to change via GH gui though :(

[drammock]

@larsoner I pushed a commit to update the needs_sphinx variable, and broke this PR in a way I've never seen before: linter CI says "error during mergeable check" and circle says fatal: Not possible to fast-forward, aborting. But AFAICT there is no conflict with upstream... GH glitch?

[larsoner]

GitHub is having a really bad day

https://www.githubstatus.com/