* Puts the docs in order, so that contributors aren't jumping to the
middle of the page to install system tools, then back to the top to
clone Firefox.
* Removes docs on MacPorts since it's being removed in bug 1688263.
* Removes step to manually install brew packages since that happens
automatically in bootstrap now.
* Simplifies mercurial installation docs
* Removes unnecessary mozconfig-tweaking instructions
* Removes almost-always-unnecessary DEFINE and troubleshooting
information.
Differential Revision: https://phabricator.services.mozilla.com/D102973