Add back to top and year

[thinkall]

Why are these changes needed?

• Add year to the event date as the events are not in the same year
• Add back to top to the end of each section as the readme is very long

Related issue number

Checks

• 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.

[BeibinLi]

LGTM.

One question: why do we need so many "back to top" button? I think 1 or 2 buttons are good enough. People can always press the "HOME" key to get to the top (unless they are using cellphone, etc.)

[manish7017]

I think adding a Table of Contents could help with navigation.