Update "3 factors" and "unmoderated" sections of the Usability Testing handbook page

Why is this change being made?

• Move the 3 factors out of the table
  • Make easier to read and use as task list
  • Better group each section and enrich with more information
• Move the 3 factors below the list so that it could be followed uninterrupted
• Enriching the section on UserTesting.com native metrics to better explain native vs our metrics
• Adding a section about Short tests to save resources & company money when tests are short

🌈 Related to improvements in #10465 (closed)

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 one of the people listed in the "Maintained by" section in on the page being edited.
  • 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. (this requirement has been removed pending identification of a new DRI for the handbook)