Add collaboration redirects (zed-industries#43471)

Redirect:

https://zed.dev/docs/collaboration ->
https://zed.dev/docs/collaboration/overview
https://zed.dev/docs/channels ->
https://zed.dev/docs/collaboration/channels

Release Notes:

- N/A

Author: JosephTLyons

docs/book.toml

@@ -56,6 +56,10 @@ enable = false
 "/model-improvement.html" = "/docs/ai/ai-improvement.html"
 "/ai/temperature.html" = "/docs/ai/agent-settings.html#model-temperature"
 
+# Collaboration
+"/channels.html" = "/docs/collaboration/channels.html"
+"/collaboration.html" = "/docs/collaboration/overview.html"
+
 # Community
 "/community/feedback.html" = "/community-links"
 "/conversations.html" = "/community-links"