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/authentication.md

@@ -4,7 +4,7 @@ Signing in to Zed is not required. You can use most features you'd expect in a c
 
 ## What Features Require Signing In?
 
-1. All real-time [collaboration features](./collaboration.md).
+1. All real-time [collaboration features](./collaboration/overview.md).
 2. [LLM-powered features](./ai/overview.md), if you are using Zed as the provider of your LLM models. To use AI without signing in, you can [bring and configure your own API keys](./ai/llm-providers.md#use-your-own-keys).
 
 ## Signing In