_content/doc: add Go 1.20 release notes

Copied from the main Go repo at CL 462286.
Deleted there in CL 464055.

For golang/go#54202.

Change-Id: I9effbceb28325c4d0394b6931b5ba48f050ea380
Reviewed-on: https://go-review.googlesource.com/c/website/+/464075
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>