add link to matrix space

mkdocs.yml

@@ -41,6 +41,7 @@ nav:
   - styleguide.md
   - faq.md
   - 'Map': 'https://map.events.ccc.de/camp/2023/map/'
+  - 'Matrix Chat': 'https://matrix.to/#/#camp2023:events.ccc.de'
   - 'Livestreams': 'https://streaming.media.ccc.de'
   - 'Recordings': 'https://media.ccc.de'
   - 'Principles of our event and community': 'https://help.ccc.de/principles.en.html'