add trigger design documentation

This MR tries to add some documentation about the overall trigger design, hardware architecture, and its boundary conditions.

@sstahl @cofitzpa @mvesteri would appreciate a review

@neufeld could you check I didn't mess up anything related to the online parts?

@lcalefic @ascarabo @valukash as discussed with some of you would be good to have feedback how this fits with the relevant starterkit documentation