Bump pymdown-extensions from 10.20.1 to 10.21 (#3680)

Bumps [pymdown-extensions](https://github.com/facelessuser/pymdown-extensions) from 10.20.1 to 10.21.
- [Release notes](https://github.com/facelessuser/pymdown-extensions/releases)
- [Commits](https://github.com/facelessuser/pymdown-extensions/commits/10.21)

---
updated-dependencies:
- dependency-name: pymdown-extensions
  dependency-version: '10.21'
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements-docs.txt

@@ -1,6 +1,6 @@
 mkdocs==1.6.1
 mkdocs-material==9.7.5
-pymdown-extensions==10.20.1
+pymdown-extensions==10.21
 mike==2.1.3
 mkdocs-git-revision-date-localized-plugin==1.5.1
 mkdocs-git-committers-plugin-2==2.5.0