Update AgBench documentation

[gagb]

What is the doc issue?

The README of agbench suggests creating ENV.json but the code uses ENV.yaml.
We need to improve this. I can help with this after I can run the bench (e.g., see #5949)

Link to the doc page, if applicable

https://github.com/microsoft/autogen/tree/main/python/packages/agbench

[gagb]

Also uv sync does not work with agbench. We need to improve the package structure.

[ekzhu]

I have a separate PR for improving AG Bench #5776 to help with some metrics like latency and better data analysis