GitHub now nicely generates a documentation site for us at
http://brew.sh/brew based on our docs folder. Optimise the output of
this and the GitHub docs directory for readability and the various user
groupings.

added 'CLT' abbreviation after 'Command Line Tools' (so that if someone follows link to footnote#3, the meaning of 'CLT' in footnote is clear)

Not quite a mass replacement as I've used OS X and Mac OS X where
describing specific older versions and added compatibility methods
for things in the DSL.

This document lists Tigerbrew and Linuxbrew as interesting forks, not
branches (they used to be "Interesting Branches (aka forks)" but that
changed in b7455469), so "Interesting Taps & Branches" is a more fitting
title.

The term "branches" is also presumably confusing to folks with a shallow
understanding of GitHub and/or git who may not realize how GitHub forks
and git branches are related.

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outdated or unneeded documentation, add some missing `Formula` API.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>

Closes Homebrew/homebrew#35529.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Closes Homebrew/homebrew#33211.