CI: (deps): Bump pydata-sphinx-theme from 0.16.1 to 0.21.0 in /ci-dev

Bumps [pydata-sphinx-theme](https://github.com/pydata/pydata-sphinx-theme) from 0.16.1 to 0.21.0.
- [Release notes](https://github.com/pydata/pydata-sphinx-theme/releases)
- [Changelog](https://github.com/pydata/pydata-sphinx-theme/blob/main/RELEASE.md)
- [Commits](pydata/pydata-sphinx-theme@v0.16.1...v0.21.0)

---
updated-dependencies:
- dependency-name: pydata-sphinx-theme
  dependency-version: 0.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

ci-dev/doc_requirements.txt

@@ -1,5 +1,5 @@
 sphinx==9.0.4
-pydata-sphinx-theme==0.16.1
+pydata-sphinx-theme==0.21.0
 sphinx-design==0.7.0
 sphinx-gallery==0.21.0
 myst-parser==5.1.0