Update Handbook with new POC content

A new doc has been developed for managed POC's: https://docs.google.com/document/d/1anbNik_H_XDHJYyZPkfr4_6wvIXgqbwvWynN9v9tj2E/edit#

Should replace this outdated page in the handbook: https://about.gitlab.com/handbook/sales/POC/