Update pymdown-extensions requirement from ~=10.15 to ~=10.16

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/10.15...10.16)

---
updated-dependencies:
- dependency-name: pymdown-extensions
  dependency-version: '10.16'
  dependency-type: direct:production
...

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

docs/requirements.txt

@@ -27,7 +27,7 @@ jinja2~=3.1
 markdown~=3.8
 mkdocs-material-extensions~=1.3
 pygments~=2.19
-pymdown-extensions~=10.15
+pymdown-extensions~=10.16
 
 # Requirements for plugins
 colorama~=0.4