updated the list of chat channels and added some general stuff

[caggles]

changes include:

• moved some guidance on how to ask for help on rocketchat to the rocketchat docs instead of the onboarding one
• removed any references to the number of users on rocketchat, since that's gonna be out of date real fast!
• grabbed some of the language that bev used in the onboarding doc to move into these docs as well.
• added a bunch of the (newer?) channels on rocketchat that people should know about
• removed the reference to devops-operations (seemed weird to me to outline the existence of one specific channel that we don't really want the community to use, but let me know if you disagree about that change)

[patricksimonian]

oops i accidently took this out of draft state. sorry Cailey!

[caggles]

ready for review!