A new human-in-the-loop section in AgentChat tutorial

[ekzhu]

We should also have a separate page in the tutorial to show human-in-the-loop, and migrate the content from https://microsoft.github.io/autogen/dev/user-guide/agentchat-user-guide/tutorial/teams.html#pausing-for-user-input to that new section. In addition, also show how to use the built-in user proxy agent to achieve a different approach of human in the loop.

Originally posted by @ekzhu in #4735 (comment)