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...2.0.0)

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

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

requirements/documentation.txt

@@ -5,5 +5,5 @@ myst-parser[linkify]>=0.14,<2.1
 sphinx-autobuild==2021.3.14
 sphinx-autodoc-typehints>=1.11,<1.26
 sphinx-copybutton>=0.2,<1
-sphinx-rtd-theme>=1,<2
+sphinx-rtd-theme>=1,<3
 sphinxext-opengraph>=0.4,<1