Bump sphinx-gallery from 0.14.0 to 0.15.0 in /requirements (#348)

Bumps [sphinx-gallery](https://github.com/sphinx-gallery/sphinx-gallery) from 0.14.0 to 0.15.0.
- [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@v0.14.0...v0.15.0)

---
updated-dependencies:
- dependency-name: sphinx-gallery
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements/requirements_doc.txt

@@ -6,7 +6,7 @@ notfound==1.0.2
 pypandoc==1.12
 pyvista==0.42.3
 sphinx-copybutton==0.5.2
-sphinx-gallery==0.14.0
+sphinx-gallery==0.15.0
 sphinx-notfound-page==1.0.0
 trame==3.3.0
 vtk==9.3.0