feat(cmmn): extend cmmn reference

- adds concepts for:
  - milestones
  - plan items / plan item definitions
  - entry / exit criteria
  - plan item lifecycles

- reformulates parts of the existing docs

related to CAM-2780/CAM-2467
chore(cmmn): improve element overview

related to CAM-2780
chore(cmmn): correct cmmn xml

related to CAM-2780