Move style guide

RFC 3309 moved the style guide to `rust-lang/rust`. In addition, the
link from
[RFC 2436](https://rust-lang.github.io/rfcs/2436-style-guide.html) to
the style guide does not seem to have ever worked, and the style guide
has never been available anywhere in rendered form.

Update RFC 2436 minimally to point to the rendered style guide.

Author: joshtriplett