Bump sphinx-rtd-theme from 0.5.2 to 2.0.0 in /cmake/tribits/doc/sphinx

Bumps [sphinx-rtd-theme](https://github.com/readthedocs/sphinx_rtd_theme) from 0.5.2 to 2.0.0.
- [Changelog](https://github.com/readthedocs/sphinx_rtd_theme/blob/master/docs/changelog.rst)
- [Commits](readthedocs/sphinx_rtd_theme@0.5.2...2.0.0)

---
updated-dependencies:
- dependency-name: sphinx-rtd-theme
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

cmake/tribits/doc/sphinx/requirements.txt

@@ -1,2 +1,2 @@
 Sphinx==4.0.2
-sphinx-rtd-theme==0.5.2
+sphinx-rtd-theme==2.0.0