docs: bump pymdown-extensions from 10.12 to 10.21.2 in /docs (#10)

Bumps [pymdown-extensions](https://github.com/facelessuser/pymdown-extensions) from 10.12 to 10.21.2.
- [Release notes](https://github.com/facelessuser/pymdown-extensions/releases)
- [Commits](facelessuser/pymdown-extensions@10.12...10.21.2)

---
updated-dependencies:
- dependency-name: pymdown-extensions
  dependency-version: 10.21.2
  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]

docs/requirements.txt

@@ -1,5 +1,5 @@
 mkdocs==1.6.1
 mkdocs-material==9.7.6
-pymdown-extensions==10.12
+pymdown-extensions==10.21.2
 pillow==12.2.0
 cairosvg==2.9.0