_content/doc: add Go 1.18 release notes

Copied from Go commit 2cb9e116d39176bd8a93609c63b175ac09eacea9 CL 388454.
Deleted there in CL 388515.

Updates golang/go#47694

Change-Id: I5f26ce2fc6ec96c5902dbe5838689fd139bcd0e5
Reviewed-on: https://go-review.googlesource.com/c/website/+/388556
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Alex Rakoczy <alex@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Run-TryBot: Carlos Amedee <carlos@golang.org>
Auto-Submit: Carlos Amedee <carlos@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Carlos Amedee <amedee@google.com>