Arch Linux has official support for the `salt` package (including
dependencies), update installation instructions to reflect this change.
Related, restructure notes about installing development builds from AUR.
Since Arch Linux has officially moved over to `systemd` as their service
manager, the Arch Installation / Post-installation steps under the installation
section should also reflect this change
Since RTD build the latest docs as the default available docs having the
installation instructions along with the rest of the docs is really the
best place for them to live.
Fixes#762Fixes#763