Update Problem validation research template

This MR adds simple but important comments about the goal of the Problem Validation issue template. Today, we use two different problem validation issues: one for the PM to articulate the problem, and another one for the researcher to execute the study. I want to bring clarity in the template description about what each template does.

This proposal also minimally updates the markdown formatting to improve the information hierarchy.

Related to gitlab!73690 (merged) and gitlab-com/www-gitlab-com!93265 (merged)