Add "unlearning" section to Engineering handbook

This idea come from a discussion from Contribute, a VPE office hours, and a coffee break with @devin and his S.O. Amanda who are both interested in company culture.

Devin in particular came from a polar opposite corporate environment. For instance, he said that as a prior company individual contributors always felt like "pawns in a manager's game" and it took him some time to understand and appreciate (and trust) how we endeavor to do things differently at GitLab.

I thought we'd try to make this explicit and get it in the handbook. Closes: #4597 (closed)