doc: update installation instructions

Clarify that GOROOT should only be set when using a custom install path.
Remove NetBSD from binary install page (we don't provide binaries).
Remove "What's next" links from installation instructions.
Emphasize "How to Write Go Code" page.

Fixes issue 6613.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/28700043