Python: Improve the getting started experience

[markwallace-microsoft]

To create an agent, should developers use client.create_agent, or should they make ChatAgent directly? The quickstart and tutorials show both approaches:
https://learn.microsoft.com/en-us/agent-framework/tutorials/agents/run-agent?pivots=programming-language-python
https://learn.microsoft.com/en-us/agent-framework/tutorials/quick-start?pivots=programming-language-python

Providing both options is causing confusion. The purpose of this task is to rethink this so we simplify the getting started experience.

Some comments:

• I'm confused as to the best practice.
• (I also think that quickstart for Python might be a bit intimidating, given it assumes developers understand async context managers).
• Learn how to create and run an AI agent using Agent Framework
• I personally found creating agents from clients counterintuitive and not favoring agent first design in multi-agent scenarios. I've converted all my samples to use ChatAgent directly.

[ekzhu]

Add another recent issue: #1329 and subsequent #1525 on create_agent to get_agent renaming