Make markdown tutorials for ChatAgent

[jjyaoao]

Description

Add documentation for core class ChatAgent.

Motivation and Context

Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.
You can use the syntax close #15213 if this solves the issue #15213

• I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)
• Example (update in the folder of example)

Implemented Tasks

• Subtask 1
• Subtask 2
• Subtask 3

Checklist

Go over all the following points, and put an x in all the boxes that apply.
If you are unsure about any of these, don't hesitate to ask. We are here to help!

• I have read the CONTRIBUTION guide. (required)
• My change requires a change to the documentation.
• I have updated the tests accordingly. (required for a bug fix or a new feature)
• I have updated the documentation accordingly.

[dandansamax]

Hi. Thank you so much for working on documents and we appreciate your contribution. The document looks clear. Here are Just some advises.

In my opinion, the documents in the get_started section should introduce the module as simple, stupid as possible. We just write something for new users to quickly understand the basic idea of the module and to get started. The detailed information can be checked in the API docs and the code. Thus, we need more intuitive description with examples in a real productive situation.

[Obs01ete]

This tutorials are very valuable as they are. Let's merge them. Later we will rework the tutorials to be more simplistic and less technical.

[lightaime]

Thanks @jjyaoao for the PR. I will look into it soon!

@Obs01ete There are some small flaws. Please wait for my review.