Deprecate google group

[meiqimichelle]

We don't use the Code for Tucson google group. We should:

• [ ] send an email suggesting people on that group move to Meetup and Slack
• [x] update the website to link to ^^ and remove the google group
• [ ] shut it down

[meiqimichelle]

Switched the links on the site to go to meetup instead of google group with this PR