docs(ai): add AI Chat with useChat guide

Comprehensive guide covering:
- Quick start with chatTask + TriggerChatTransport
- Backend patterns: simple (return streamText), complex (pipeChat),
  and manual (task + ChatTaskPayload)
- Frontend options: dynamic tokens, extra data, self-hosting
- ChatTaskPayload reference
- Added to Writing tasks navigation near Streams

Co-authored-by: Eric Allam <eric@trigger.dev>

Author: cursoragent