Bump readthedocs-sphinx-ext from 2.1.5 to 2.1.9 (#1397)

Bumps [readthedocs-sphinx-ext](https://github.com/readthedocs/readthedocs-sphinx-ext) from 2.1.5 to 2.1.9.
- [Release notes](https://github.com/readthedocs/readthedocs-sphinx-ext/releases)
- [Commits](readthedocs/readthedocs-sphinx-ext@2.1.5...2.1.9)

---
updated-dependencies:
- dependency-name: readthedocs-sphinx-ext
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

docs/requirements.txt

@@ -1,4 +1,4 @@
 sphinx==4.5.0
-readthedocs-sphinx-ext==2.1.5 # ??
+readthedocs-sphinx-ext==2.1.9 # ??
 sphinx-rtd-theme
 myst-parser==0.17.2