doc: expand THE_CASK_LANGUAGE, shrink CONTRIBUTING

following up on #3094
- expand `THE_CASK_LANGUAGE.md` to more fully specify the Cask language
- add sections: "Casks Are Ruby Classes" and "The Cask Language Is Declarative"
- import content from `FAQ.md` to `THE_CASK_LANGUAGE.md`, create multiple sections
  listing all stanzas according to category.
- document which stanzas are permitted multiple times
- add previously undocumented `before_install` and `before_uninstall`
- doc that `Hardware::CPU.is_64_bit?` (and 32-bit) are acceptable in conditionals
- doc that `:target` may contain an absolute path (this works more reliably after #3075)
- doc that `uninstall` is optional in the DSL, but required by the community
- link examples
- standardize mixed language on "stanza" over "field"
- further shrink `CONTRIBUTING.md` by deleting rarely used stanzas
  and referencing `THE_CASK_LANGUAGE.md`