Add joining sentry to support onboarding checklist

Closes #899 (closed)

How to use Sentry is already in the Knowledge Base here

I chose not to add a link to the knowledge base in the onboarding checklist because onboarding is overwhelming enough and I would call this an advanced topic. The goal with this MR is just to make sure that all new SE's are added properly so that later when they want to access Sentry, they are already have an account and they are added to the groups.

And that they have already learned the word Sentry and that it is a place they can log in to. That might sound like a joke but I am serious, I have been seeing people mention Sentry on Slack and assumed it is something that more senior people have access to and never investigated further.