Add more social links (#765)

And update social.md old youtube link with new one.

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

social.md

@@ -26,7 +26,7 @@ List of channels actively maintained by JMS in alphabetical order:
 - Mastodon:  https://hachyderm.io/@ProjectJupyter
 - Slack JupyterLab: https://jupyterlabworkspace.slack.com
 - X/Twitter Jupyter: https://twitter.com/ProjectJupyter
-- YouTube: https://youtube.com/@ipython
+- YouTube: https://youtube.com/@projectjupyter
 
 List of channels considered inactive but managed by the JMS: