Update sphinx-rtd-theme requirement

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...3.0.2)

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

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

requirements/docs.txt

@@ -2,7 +2,7 @@
 # core dependencies
 sphinx>=4.2.0, <8.0.0
 sphinx-autodoc-typehints>=1.12.0, <1.21  # limited due to https://github.com/tox-dev/sphinx-autodoc-typehints/issues/260
-sphinx-rtd-theme>=1.0.0, <2.0.0
+sphinx-rtd-theme>=1.0.0, <4.0.0
 sphinxcontrib-apidoc>=0.3.0, <0.5.0
 sphinxcontrib-bibtex>=2.1.0, <3.0.0
 myst-parser>=1.0, <3.0