go/doc/comment: parse and print explicit links

go/doc/comment: parse and print explicit 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 explicit links, like:

	Visit the [Go home page].

	[Go home page]: https://go.dev

For #51082.

Change-Id: If8104e45558314dae0346df614b03d5664421cf1
Reviewed-on: https://go-review.googlesource.com/c/go/+/397282
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>