Initial commit of Rec Room docs in Markdown format

- Will eventually move docs to MDN App Center - Quick Start, but
  we need them on GitHub as well.
Feels a little odd to be throwing around info and links about Hosted/Privileged/Packaged Apps in this paragraph before the section explaining that there are different types of applications.  Not sure how to re-word/organize this yet but will come back to this and PR something.