deps: bump sphinx-gallery from 0.18.0 to 0.20.0 (#119)

Bumps [sphinx-gallery](https://github.com/sphinx-gallery/sphinx-gallery) from 0.18.0 to 0.20.0.
- [Release notes](https://github.com/sphinx-gallery/sphinx-gallery/releases)
- [Changelog](https://github.com/sphinx-gallery/sphinx-gallery/blob/master/CHANGES.rst)
- [Commits](sphinx-gallery/sphinx-gallery@v0.18.0...v0.20.0)

---
updated-dependencies:
- dependency-name: sphinx-gallery
  dependency-version: 0.20.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

docs/requirements.txt

@@ -240,7 +240,7 @@ sphinx==8.1.3
     #   sphinx-gallery
 sphinx-copybutton==0.5.2
     # via pyopenms_viz (pyproject.toml)
-sphinx-gallery==0.18.0
+sphinx-gallery==0.20.0
     # via pyopenms_viz (pyproject.toml)
 sphinxcontrib-applehelp==2.0.0
     # via sphinx