Add resources about A/B testing and product design to the Growth UX handbook page

Why is this change being made?

For product team outside of Growth A/B testing can be quite hard to grasp, how/why the Growth team is running A/B tests, how Growth designers are implementing it in their daily workflow... Growth designers should provide resources for other product team that would be interested in running A/B test.

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][transparency]
• 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][mr-buddies-slack].
  • 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.