Bump pymdown-extensions from 10.16.1 to 10.21.3 in /docs

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

@@ -49,7 +49,7 @@ pygments==2.19.2 ; python_version >= "3.10" and python_version < "3.15"
 pyinstaller-hooks-contrib==2026.0 ; python_version >= "3.10" and python_version < "3.15"
 pyinstaller==6.19.0 ; python_version >= "3.10" and python_version < "3.15"
 pylint==4.0.5 ; python_version >= "3.10" and python_version < "3.15"
-pymdown-extensions==10.16.1 ; python_version >= "3.10" and python_version < "3.15"
+pymdown-extensions==10.21.3 ; python_version >= "3.10" and python_version < "3.15"
 pync==2.0.3 ; python_version >= "3.10" and python_version < "3.15" and sys_platform == "darwin"
 pytest-cov==7.0.0 ; python_version >= "3.10" and python_version < "3.15"
 pytest-describe==3.1.0 ; python_version >= "3.10" and python_version < "3.15"