Bump pymdown-extensions from 10.21.2 to 10.21.3 in /site

Bumps [pymdown-extensions](https://github.com/facelessuser/pymdown-extensions) from 10.21.2 to 10.21.3.
- [Release notes](https://github.com/facelessuser/pymdown-extensions/releases)
- [Commits](facelessuser/pymdown-extensions@10.21.2...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]

site/requirements.txt

@@ -8,4 +8,4 @@ mkdocs-minify-plugin==0.8.0
 mkdocs-monorepo-plugin==1.1.2
 mkdocs-redirects==1.2.3
 mkdocs-rss-plugin==1.19.0
-pymdown-extensions==10.21.2
+pymdown-extensions==10.21.3