FIX: Pin sphinx-gallery

in MNE-BIDS doc/conf.py we have:

```
sphinx_gallery_conf["within_subsection_order"] = "mne_bids.utils._example_sorter"
```

But AFAICT this functionality wasn't workign prior to sphinx-gallery/sphinx-gallery#1391 , which was included in the release of sphinx-agallery v0.19 (released Feb 12 2025).

https://sphinx-gallery.github.io/stable/changes.html#v0-19-0

Given that v0.19 was released less than 1 year ago, I think it is reasonable to assume that devs/contributors might have a sphinx-gallery version < 0.19 installed locally, in which case building the mne-bids docuemntation will fail, but the reason/solution won't be explicit.

So I think that it is best to be explicit about the minimum required sphinx-gallery version we require to be able to successfully build our docs.

Author: scott-huberty

pyproject.toml

@@ -58,7 +58,7 @@ doc = [
   "seaborn",
   "sphinx-copybutton",
   "sphinx>=7.4.7",
-  "sphinx_gallery",
+  "sphinx_gallery>=0.19.0",
 ]
 # Dependencies for using all mne_bids features
 full = [