Update API reference to rename chat to assistant terminology

[mintlify]

This PR updates the API reference section to rename "chat" to "assistant" throughout the documentation. This includes:

1. Updating endpoint references from /chat/topic to /assistant/topic and /chat/message to /assistant/message
2. Changing all instances of "Chat API" to "Assistant API"
3. Renaming "create a chat topic" to "create an assistant topic"
4. Moving file paths from api-reference/chat/ to api-reference/assistant/
5. Updating the navigation structure in docs.json to reflect these changes