CI: (deps): Bump pydata-sphinx-theme from 0.15.2 to 0.15.4 in /ci

Bumps [pydata-sphinx-theme](https://github.com/pydata/pydata-sphinx-theme) from 0.15.2 to 0.15.4.
- [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@v0.15.2...v0.15.4)

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

Signed-off-by: dependabot[bot] <[email protected]>

ci/doc_requirements.txt

@@ -1,5 +1,5 @@
 sphinx==7.3.7
-pydata-sphinx-theme==0.15.2
+pydata-sphinx-theme==0.15.4
 sphinx-design==0.5.0
 sphinx-gallery==0.16.0
 myst-parser==3.0.1