Update sphinx-rtd-theme requirement from ~=1.1 to ~=1.2 in /docs

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@1.1.0...1.2.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

docs/requirements.txt

@@ -1,3 +1,3 @@
 # RTD pins these by default to ancient versions
 sphinx ~= 5.0
-sphinx-rtd-theme ~= 1.1
+sphinx-rtd-theme ~= 1.2