_content/doc: add Go 1.19 release notes

Copied from the main Go repo at CL 417534.
Deleted there in CL 420418.

For golang/go#51400.

Change-Id: I32b1464638f3aa8fee6dad65aa8331da0c33c50c
Reviewed-on: https://go-review.googlesource.com/c/website/+/420417
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>