doc: add an editor guide

The Go ecosystem provides many tools to make Go
development more productive and seamless. Document
the availability of the editor plugins and IDEs,
add an overview of feature support and screencasts.

Updates #20398.
Updates #20402.
Updates #20399.
Updates #20401.
Updates #20569.

Change-Id: I0c6cb48eb4e3848807aaad78390493e14f097916
Reviewed-on: https://go-review.googlesource.com/45772


Reviewed-by: Steve Francia <spf@golang.org>