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.
- [Release notes](https://github.com/readthedocs/sphinx_rtd_theme/releases)
- [Changelog](https://github.com/readthedocs/sphinx_rtd_theme/blob/master/docs/changelog.rst)
- [Commits](readthedocs/sphinx_rtd_theme@0.5.0...1.0.0)

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

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

requirements/documentation.txt

@@ -8,4 +8,4 @@ sphinx-autodoc-typehints>=1.11,<1.13
 sphinx-copybutton>=0.2,<1
 sphinx-markdown-tables>=0.0.15,<0.1
 sphinx-panels>=0.5,<0.7
-sphinx-rtd-theme>=0.5,<1
+sphinx-rtd-theme>=0.5,<2