"quick start" wiki pages for the 3 major platforms

[lkuper]

The "Getting started" page is getting too long and confusing. It would be nice to have separate instructions for Mac/Linux/Windows. We can still have a more detailed page for when the quick start page for one's own platform doesn't suffice.

[brson]

Our build process is considerably simpler now than it was back then. I think we can close this?