Restructure collaboration docs (#43464)

Overview
    - Channels
    - Private calls

---

Up next would be to 

- [ ] Update any zed.dev links to point to items in this structure
- [ ] Update content in these docs (would prefer to do that in a
separate PR from this one)

Release Notes:

- N/A

docs/src/SUMMARY.md

@@ -34,8 +34,9 @@
 - [Command-line Interface](./command-line-interface.md)
 - [Outline Panel](./outline-panel.md)
 - [Code Completions](./completions.md)
-- [Channels](./channels.md)
-- [Collaboration](./collaboration.md)
+- [Collaboration](./collaboration/overview.md)
+  - [Channels](./collaboration/channels.md)
+  - [Private Calls](./collaboration/private-calls.md)
 - [Git](./git.md)
 - [Debugger](./debugger.md)
 - [Diagnostics](./diagnostics.md)