chore(deps): bump pydata-sphinx-theme in /requirements

Bumps [pydata-sphinx-theme](https://github.com/pydata/pydata-sphinx-theme) from 0.15.3 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@v0.15.3...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] <support@github.com>

Author: dependabot[bot]

requirements/requirements-docs.txt

@@ -1,3 +1,3 @@
-pydata-sphinx-theme==0.15.3
+pydata-sphinx-theme==0.15.4
 Sphinx==7.3.7
 sphinx-copybutton==0.5.2