chore(deps): update sphinx-rtd-theme requirement in /requirements

Updates the requirements on [sphinx-rtd-theme](https://github.com/readthedocs/sphinx_rtd_theme) to permit the latest version.
- [Changelog](https://github.com/readthedocs/sphinx_rtd_theme/blob/master/docs/changelog.rst)
- [Commits](readthedocs/sphinx_rtd_theme@1.0.0...3.0.1)

---
updated-dependencies:
- dependency-name: sphinx-rtd-theme
  dependency-type: direct:production
...

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

requirements/documentation.txt

@@ -6,5 +6,5 @@ sphinx-autobuild>=2024
 sphinx-autodoc-typehints>=1.25
 sphinx-copybutton>=0.5,<1
 sphinxext-opengraph>=0.7
-sphinx-rtd-theme>=1,<3
+sphinx-rtd-theme>=1,<4
 sphinx-sitemap>=2.5.1