chore(deps): update sphinx-rtd-theme requirement (#499)

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@1.3.0...2.0.0)

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

Signed-off-by: dependabot[bot] <[email protected]>
Signed-off-by: Jan Kowalleck <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

docs/requirements.txt

@@ -1,4 +1,4 @@
 m2r2>=0.3.2
 sphinx>=7.2.6,<8
 sphinx-autoapi>=3.0.0,<4
-sphinx-rtd-theme>=1.3.0,<2
+sphinx-rtd-theme>=2.0.0,<3