remove github, dev.to, gitter.im, added codeberg & new matrix address

mkdocs.yml

@@ -131,8 +131,6 @@ extra:
   social:
     - icon: simple/codeberg
       link: https://codeberg.org/stiebke25
-    - icon: fontawesome/brands/dev
-      link: https://dev.to/stiebke
     - icon: fontawesome/brands/bluesky
       link: https://bsky.app/profile/stiebke.bsky.social
     - icon: fontawesome/brands/mastodon
@@ -145,12 +143,12 @@ extra:
       link: https://www.xing.com/profile/Stephan_Hadan
     - icon: fontawesome/brands/linkedin
       link: https://de.linkedin.com/in/stiebke2010
-    - icon: fontawesome/brands/gitter
-      link: https://matrix.to/#/@stiebke:gitter.im
+    - icon: simple/matrix
+      link: https://matrix.to/#/@stiebke25:tchncs.de
     - icon: fontawesome/brands/signal-messenger
       link: https://stephan.hadan.de/contact/#signal
-    - icon: fontawesome/brands/slack
-      link: https://hadanit.slack.com/team/U04BLKQF2M8
+    #- icon: fontawesome/brands/slack
+    #  link: https://hadanit.slack.com/team/U04BLKQF2M8
     - icon: simple/pixelfed
       link: https://pixelfed.de/users/stiebke
     - icon: fontawesome/solid/signal