Update sphinx-rtd-theme requirement from <2.0.0 to <4.0.0

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@0.1.8...3.0.2)

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

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

docs/requirements.txt

@@ -1,5 +1,5 @@
 sphinx == 7.2.4
-sphinx-rtd-theme < 2.0.0
+sphinx-rtd-theme < 4.0.0
 sphinx-copybutton == 0.5.2
 autodoc_pydantic == 2.2.0
 furo