shrink CONTRIBUTING.md to 1/4 its current size

Rationale:
- lengthy CONTRIBUTING.md may itself discourage new Cask authors
- it is doubtful that everyone reads to the end, where there
  are important notes about squashing.

CONTRIBUTING.md reduced from 7947 words to 1927 words.

This was accomplished principally by spinning out "details" sections
into a new reference file `doc/THE_CASK_LANGUAGE.md`, but also by
ruthless re-organization, removal of duplication, and trimming of
phrases. Content remains 99% identical, though a few typos were
caught along the way, and a table of contents was added.