Update readme and AutoGen docs

[thinkall]

Why are these changes needed?

Update AutoGen docs and Fabric link in readme.

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/FLAML/. See https://microsoft.github.io/FLAML/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[sonichi]

Would you like to update https://microsoft.github.io/FLAML/docs/Use-Cases/Autogen#notebook-examples as well? Shall we keep the two lists in the two pages or use a link to the example page in the use case page?

[thinkall]

I think keep the two lists is better for reader experience. Added comments before the lists to remind alignment between them.
Also added the new arxiv link to it.