Update sphinx-rtd-theme requirement from <3.0,>=1.0 to >=1.0,<4.0

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@1.0.0...3.0.0)

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

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

docs/requirements-docs.txt

@@ -1,5 +1,5 @@
 # Requirements to build hpcss' documentation.
 
 sphinx >=5.0, <9.0
-sphinx-rtd-theme >=1.0, <3.0
+sphinx-rtd-theme >=1.0, <4.0
 tomlkit >=0.1, < 1.0