[racket-dev] Getting Started & install instructions

Hi all,

I've long thought that the Getting Started page in the documentation is
too sparse, so I discussed with Ryan and added some more content to it.
The intent is to try to answer some frequent questions that beginners
ask (e.g., "do I want the `racket` or the `drracket` executable?").

Ideally, I'd like to link to platform-specific installation instructions
for Racket, but I couldn't find any on the Racket home page. Do we have
a page that we can link to for this purpose?

Cheers,
Asumu