[Tree Collection] Add documentation

[Tihomir971]

• For example api for tree-collection isn't documented. It has lot of useful functions that i created in my code and now I find out that you already have them exposed :(
• Collection should be in llm friendly format (not link to web page)
• Language LLM (llms-svelte.txt) is better to be index file for individual components including shared like Collection

[anubra266]

@Tihomir971 Feel free to open a PR

[segunadebayo]

I've gone ahead and added some documentation for it in the docs
https://zagjs.com/guides/collection#tree-collection