Improve doc in tutorial/conversation-patterns and customized_speaker_selection

[yiranwu0]

Some developers are wondering how to make the system behaviors more deterministic, but have a hard time finding relevant docs (customize groupchat speaker selection, nested chats). This PR tries to improve relevant docs:

• Reorganize https://microsoft.github.io/autogen/docs/topics/groupchat/customized_speaker_selection, add pointer to stateflow blogpost.
• Add an overview paragraph in https://microsoft.github.io/autogen/docs/tutorial/conversation-patterns, and pointers to customized_speaker_selection and stateflow (which is not in this doc)
• Change my username from kevin666aa to yiranwu0 in doc.

Next step: The tutorial/conversation-patterns is too long. I will make a new PR trying to move a few things to User Guide so that this section is easier to read.

Why are these changes needed?

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• [ ] I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/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.