Add more social links

Author: jtpio

_layouts/default.html

@@ -41,6 +41,10 @@
     url: https://github.com/jupyter/
   - title: Mastodon
     url: https://hachyderm.io/@ProjectJupyter
+  - title: X
+    url: https://x.com/ProjectJupyter
+  - title: YouTube
+    url: https://www.youtube.com/@projectjupyter
 legal:
   - title: Privacy
     url: /privacy