Update pymdown-extensions requirement from ~=10.18 to ~=10.19

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/10.18...10.19.1)

---
updated-dependencies:
- dependency-name: pymdown-extensions
  dependency-version: 10.19.1
  dependency-type: direct:production
...

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

docs/requirements.txt

@@ -27,7 +27,7 @@ jinja2~=3.1
 markdown~=3.10
 mkdocs-material-extensions~=1.3
 pygments~=2.19
-pymdown-extensions~=10.18
+pymdown-extensions~=10.19
 
 # Requirements for plugins
 colorama~=0.4