chore: remove stale TODOs and dead code

[sanjana2505006]

Removes large sections of commented-out HTML and outdated TODO metadata from community.html to improve source cleanliness.

[sanjana2505006]

Hello @jamesfredley, While reviewing pages/community.html, I saw that there were large chunks of commented-out HTML code.
This PR seeks to remove these commented-out chunks to make the source more readable. Specifically:
The old Conferences section, which was commented out with a TODO about 2020 events.
A few broken Meetup links in the User Groups section, which were commented out with a TODO about broken links.
There are no changes to functionality; this is purely a cleanup to make the source file more readable.