Handbook Support: Add correct user mapping instructions
Closes gitlab-com/support/support-team-meta#1553 (closed)
Closes gitlab-com/support/support-team-meta#1406 (closed)
@lyle if you could review, especially the correctly mapped user portion with all the script instructions and the like.
Note, basic instructions on first time setup and limitations discussed in today's meeting are in a MR to the project's ReadMe: gitlab-com/support/toolbox/dcef!1 (closed)