Docmentation for agents (#1057)

* add agent notebook and documentation

* fix bug

* set flush to True when printing msg in agent

* add a math problem in agent notebook

* remove

* header

* improve notebook doc

* notebook update

* improve notebook example

* improve doc

* improve notebook doc

* improve print

* doc

* human_input_mode

* human_input_mode str

* indent

* indent

* Update flaml/autogen/agent/user_proxy_agent.py

Co-authored-by: Chi Wang <wang.chi@microsoft.com>

* Update notebook/autogen_agent.ipynb

Co-authored-by: Chi Wang <wang.chi@microsoft.com>

* Update notebook/autogen_agent.ipynb

Co-authored-by: Chi Wang <wang.chi@microsoft.com>

* Update notebook/autogen_agent.ipynb

Co-authored-by: Chi Wang <wang.chi@microsoft.com>

* add agent doc

* del old files

* remove chat

* agent doc

* remove chat_agent

* naming

* improve documentation

* wording

* improve agent doc

* wording

* general auto reply

* update agent doc

* human input mode

* add agent figure

* update agent figure

* update agent example figure

* update code example

* extensibility of UserProxyAgent

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Co-authored-by: Chi Wang <wang.chi@microsoft.com>