Add link to problem validation research template

Why is this change being made?

This MR adds a missing link to the UX Research Problem Validation template.

@katiemacoy brought to my attention that it wasn't clear why the handbook page asked for two different problem validation issues.

• The issue on .com is about defining the problem and the level of confidence, and also the effort it will take for us to solve the problem. It is not about researching it.
• The second issue lives in the ux-research project, and is about actually researching and coordinating the effort. It uses a different template that is not linked in the handbook.

This MR intends to clarify that distinction in an easy way.

Related to gitlab-org/gitlab!73690 (merged) gitlab-org/ux-research!57 (merged)

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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