Why are these changes needed?

Add documentation and examples for AgentChat.

• Add landing page introducing AgentChat and the concepts of Agents and Teams
• Provide a migration guide in the Quick Start section, with an example of how to migrate 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)

Related issue number

#3633

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.