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.