Added further detail on Validation process

This MR provides more detail on specific expectations when conducting Validation. I propose using a single issue for problem validation. Keeping a single SSOT on a single problem will help retain context, and we can append additional detail to the validation issue's description as needed.

This MR relies on the MR in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/16595, which adds a "Problem Validation" issue template.

The problem this MR intends to help with was reducing some of the fuzziness on this existing page. Ideally, the dimensions we'd like to start with should be relatively unambiguous - similarly, the gates that issues pass through during the process should have some fairly well-defined criteria.

We should consider a "Build" issue template as well. If there's a problem validation issue to link to, we can probably focus our attention a bit more on the validated solution and less on the problem/users/journey (which was explored and defined).