Community doc: http://file.ork.redhat.com/~mramendi/BXMSDOC-6496-comm/#_jbpmpersistence 

Product doc: http://file.ork.redhat.com/~mramendi/BXMSDOC-6496-prod/#persistence-con_process-engine

PR: https://github.com/mramendi/kie-docs/pull/126 

(PR will possibly be shared with the Integration chapter later. I am looking at dropping Integration tomorrow Wendesday, if I can't I don't think it makes 7.9GA. Juggling branches is just too hard)

This is a full modularization of the persistence and transaction chapter. As discussed, assigning at the same time as SME review for a high level peer review, then we will do a polish/final peer review of all the new stuff at teh same time as QE review.