[release-branch.go1.2] doc: update installation instructions

««« CL 28700043 / 8b3699321603
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 #6613.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/28700043
»»»

R=golang-dev
CC=golang-dev
https://golang.org/cl/34950043