doc/go1.19: prefer relative links to other parts of the Go website
The Go website can be served on more than one domain (for example,
go.dev, golang.google.cn, tip.golang.org, localhost:6060, and so on).
Use relative links which work in all contexts.
For #51400.
Updates #53337.
Change-Id: I100938981447537ac242b4045929f6db8a2674c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/411974
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org> Reviewed-by: Michael Knyszek <mknyszek@google.com>