Add outcomes and activities to Phase 4 Solution Validation

Why is this change being made?

This change is being made in an attempt to clearly articulate the activities and outcomes expected from the Solution Validation phase of the Product Development Flow as described under #9098 (closed).

This change moves Validation Phase 4 - Solution validation from a detailed process description to an outcome and activity focused section. This means rather than listing how labels need to be applied it focuses on the why and the expected outcomes.

The phase template has been leveraged.

Note: We have removed the step by step in this MR but are considering linking to them as part of a 'sample flow on a different page.

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
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.

Closes #9098 (closed)