Make Chat class public: small updates + add docs

[jackmpcollins]

This is very useful for tracking chat messages and handling function call execution.

TODO

• make importable from top level
• write docs page about usage
• (maybe out of scope) enable setting/overriding output type in Chat.submit()
• (maybe out of scope) add handling for error within a function call

magentic/src/magentic/chat.py

Lines 24 to 35 in 6747d99

	class Chat:
	"""A chat with an LLM chat model.
	Examples
	--------
	>>> chat = Chat().add_user_message("Hello")
	>>> chat.messages
	[UserMessage('Hello')]
	>>> chat = chat.submit()
	>>> chat.messages
	[UserMessage('Hello'), AssistantMessage('Hello! How can I assist you today?')]
	"""