Creating this ticket to track information about the initial work on the Search 6 proof-of-concept, so that we have a ticket to reference in the initial Search 6 commits, where we worked in "exploratory" mode. I'll rewrite history to add this message to commits.

Search 6 Proof-of-concept repository: https://github.com/hibernate/hibernate-search-6-poc/

Work document: https://docs.google.com/document/d/16PAa__LsxyLZcbW3q1MvgyIznh4ZnCYLupbNMXKMRds/edit?pli=1#