Update pymdown-extensions requirement from ~=10.12 to ~=10.13

Updates the requirements on [pymdown-extensions](https://github.com/facelessuser/pymdown-extensions) to permit the latest version.
- [Release notes](https://github.com/facelessuser/pymdown-extensions/releases)
- [Commits](https://github.com/facelessuser/pymdown-extensions/compare/10.12...10.13)

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

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

docs/requirements.txt

@@ -26,7 +26,7 @@ jinja2~=3.1
 markdown~=3.7
 mkdocs-material-extensions~=1.3
 pygments~=2.19
-pymdown-extensions~=10.12
+pymdown-extensions~=10.13
 
 # Requirements for plugins
 babel~=2.16