ports.7/FILES: Expand and refactor into 3 tables

Add make.conf, CHANGES, CONTRIBUTING.Md, UPDATING, and Tools/scripts.
Refactor the FILES section of the ports reference manual into a bigger
table with three sections separated by root directory. Remove preceeding
article from all but "the big Kahuna", and root dirs where reasonable.

MFC after: 3 days
Relnotes: yes
Reported by: adamw, arrowd, linimon
Differential Revision: https://reviews.freebsd.org/D55441

(cherry picked from commit 28deec9b7e2b08b00160eb39de84ade6f27eb125)