Major revisions to wiki page (part 3)

Completes the revision work expected in #324959 (closed) - brings ./user/project/wiki/index.md much closer to CTRT in style and content, at @ericschurter's request.

Suzanne: I have been over this page so many times that I'm struggling to see the content clearly now. You have my blessing to do a full, ruthless edit. I've cleared out a lot of cruft, so hopefully the rest of the work will be clear to fresh eyes.

Related issues

Closes #324959 (closed) at last