Add MPR installation instructions
This adds instructions for installing the Airship launcher from the MPR as discussed on Discord. This package will also be in the Prebuilt-MPR shortly, where it would be already compiled, though at that point it's basically like the PPA method, so I didn't add it in the docs yet.
I haven't been able to view my changes locally as I don't have Zola installed on my system, so if someone could view everything to make sure it checks out, I'd appreciate that.