Bump sphinx-rtd-theme from 1.1.1 to 1.2.1 in /docs/source

Bumps [sphinx-rtd-theme](https://github.com/readthedocs/sphinx_rtd_theme) from 1.1.1 to 1.2.1.
- [Changelog](https://github.com/readthedocs/sphinx_rtd_theme/blob/master/docs/changelog.rst)
- [Commits](readthedocs/sphinx_rtd_theme@1.1.1...1.2.1)

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

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

Author: dependabot[bot]

docs/source/requirements.txt

@@ -1,5 +1,5 @@
 # Defining the exact versions for ReadTheDocs that will make sure things don't break
 sphinx==5.3.0
-sphinx_rtd_theme==1.1.1
+sphinx_rtd_theme==1.2.1
 readthedocs-sphinx-search==0.3.2
 recommonmark