Fix nicknames, and add instruction on it

Made nicknames for a few team members consistent; (specifically @twk3 @xyzzy ), added a guideline on it as part of the guide on how to add yourself to the team page.

I then moved that entire instruction page to the handbook/general-onboarding page, because it did not make sense to me that it was a dangling file at the top level of the repo.

\cc @brittanyr