doc/go1.18: move debug/buildinfo to core library section

It is a new package and seems a major change.

Updates #47694.

Change-Id: If854e494e28bcd1e79c99e59119755b9cb6793d6
Reviewed-on: https://go-review.googlesource.com/c/go/+/371816


Trust: Cherry Mui <cherryyz@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>