add links to stan groups (meetup, facebook groups, etc.) to "Community" page

[jgabry]

Here's the Korea group we heard about in today's meeting:

https://www.facebook.com/groups/StanKorea

and there are more Stan meetup groups we can link to.

[mitzimorris]

like events, meetup groups go stale... nonetheless - collecting a list:

https://www.meetup.com/Stan-Users-Boston-Camberville/

https://www.meetup.com/Melbourne-Stan-and-Bayesian-Inference-Meetup/ (7 past events - monthly (sic) meetups)

https://www.meetup.com/Los-Angeles-Bayesian-Statistics-STAN-Meetup/ (56 members, no events past or future)

https://www.meetup.com/bda-group/ - New York - Lauren's organizing it for now

https://discourse.mc-stan.org/t/announcing-first-dutch-stan-meetup/6479