[release-branch.go1.18] cmd/go: add links to workspaces reference and tutorial to go help work

For #45713

Change-Id: Ia2901cbfc5deb52503e74fcf9dff26a56ec582c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/389297


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
(cherry picked from commit 1eb1f621)
Reviewed-on: https://go-review.googlesource.com/c/go/+/389914
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>