build(deps): bump pymdown-extensions from 10.16.1 to 10.21.3

Bumps [pymdown-extensions](https://github.com/facelessuser/pymdown-extensions) from 10.16.1 to 10.21.3.
- [Release notes](https://github.com/facelessuser/pymdown-extensions/releases)
- [Commits](facelessuser/pymdown-extensions@10.16.1...10.21.3)

---
updated-dependencies:
- dependency-name: pymdown-extensions
  dependency-version: 10.21.3
  dependency-type: direct:production
...

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

Author: dependabot[bot]

docs/requirements.txt

@@ -20,7 +20,7 @@ paginate==0.5.7
 pathspec==0.12.1
 platformdirs==4.3.6
 Pygments==2.20.0
-pymdown-extensions==10.16.1
+pymdown-extensions==10.21.3
 python-dateutil==2.9.0.post0
 PyYAML==6.0.2
 pyyaml_env_tag==0.1