Overhaul, simplify and cleanup documentation.

Remove duplication, link to the API documentation more often,
tweak wording, add `@UniqMartin` as a maintainer, note `@jacknagel`'s
and `@adamv`'s significant past contributions to Homebrew, delete some
outdated or unneeded documentation, add some missing `Formula` API.
entries and simplify/improve `CONTRIBUTING.md`.

Closes Homebrew/homebrew#46179.

Closes Homebrew/homebrew#46618.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>