fix: pin pygments<2.20 to fix MkDocs build

pymdown-extensions 10.21 passes filename=None to Pygments 2.20.0 HtmlFormatter,
causing 'NoneType' object has no attribute 'replace' error.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: HavenDV

.github/workflows/mkdocs.yml

@@ -48,7 +48,7 @@ jobs:
         run: |
           python -m venv myenv
           source myenv/bin/activate
-          pip install mkdocs-material
+          pip install mkdocs-material 'pygments<2.20'
           mkdocs build -d ./_site
 
       - name: Upload artifact