Add documenation and examples for AgentChat

[victordibia]

What

Add documentation for AgentChat

• [ ] Add landing page introducing the concepts of Agents andTeams
• [ ] Provide a migration guide with an example of how to implement the basic stock price plot example from autogen 0.2x to AgentChat 0.4x.
• [ ] Improve current documentation on Tool Use, showing example on using LangChain Tools
• [ ] Add a set of 3 concrete examples implementing workflows with AgentChat
  • [ ] Travel Planning
  • [ ] Company Research and Report Generation (using web search and custom tools)
  • [ ] Academic Literature Review with Arxiv (using web search, and arxiv tools)

Why is this needed?

To help new users rapidly get started with AutoGen