go/doc/comment: parse and print doc links

go/doc/comment: parse and print doc links

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

Implement parsing and printing of documentation links,
like [math.Sqrt] or [*golang.org/x/text/runes.Set].

For #51082.

Change-Id: I1cc73afbac1c6568773f921ce4b73e52f17acef6
Reviewed-on: https://go-review.googlesource.com/c/go/+/397281
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>