Update sphinx-rtd-theme requirement from ~=2.0 to ~=3.0 in /docs

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@2.0.0...3.0.0)

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

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

docs/requirements.txt

@@ -1,3 +1,3 @@
 # RTD pins by default to ancient versions
-sphinx-rtd-theme ~= 2.0
+sphinx-rtd-theme ~= 3.0
 m2r2 ~= 0.3