Update sphinx-gallery requirement from ~=0.18 to ~=0.19

Updates the requirements on [sphinx-gallery](https://github.com/sphinx-gallery/sphinx-gallery) to permit the latest version.
- [Release notes](https://github.com/sphinx-gallery/sphinx-gallery/releases)
- [Changelog](https://github.com/sphinx-gallery/sphinx-gallery/blob/master/.github_changelog_generator)
- [Commits](sphinx-gallery/sphinx-gallery@v0.18.0...v0.19.0)

---
updated-dependencies:
- dependency-name: sphinx-gallery
  dependency-version: 0.19.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

docs/requirements.txt

@@ -6,7 +6,7 @@ sphinx~=8.1; python_version >= '3.10'
 sphinx-rtd-theme~=3.0
 sphinxcontrib-spelling~=8.0
 sphinxcontrib-googleanalytics~=0.4
-sphinx-gallery~=0.18
+sphinx-gallery~=0.19
 nbsphinx~=0.9
 
 -r notebooks/requirements.txt