cmd/doc: use new go/doc APIs

cmd/doc: use new go/doc APIs

[This CL is part of a sequence implementing the proposal #51082.
The design doc is at https://go.dev/s/godocfmt-design.]

Use the new per-Package go/doc API instead of the
top-level functions from go/doc. These handle links better.

For #51082.

Change-Id: I169b46d973673abdb6f126352c9f1e30f9fe1122
Reviewed-on: https://go-review.googlesource.com/c/go/+/384266
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>