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Removes large sections of commented-out HTML and outdated TODO metadata from community.html to improve source cleanliness.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

[sbglasius]

@jamesfredley we should discuss if this page is even relevant any more. The only active group is that in Madrid.

[sbglasius]

This is also not there any more 🙁

[sbglasius]

No activity since 2015 🙁

@jamesfredley should we remove?

[sbglasius]

This is also not there any more 🙁

[sbglasius]

@sanjana2505006 please see comments and rebase. Then I'll have another look so we can get this merged.

@jamesfredley What should we do about the stale user groups?

[sanjana2505006]

@sbglasius Done