Add meta descriptions to UX handbook pages

Why is this change being made?

For SEO, adds meta descriptions to:

Pages
https://about.gitlab.com/handbook/engineering/ux/ux-research/
https://about.gitlab.com/handbook/engineering/ux/ux-resources/designers-guide-to-contributing-ui-changes-in-gitlab/
https://about.gitlab.com/handbook/engineering/ux/ux-resources/designers-step-by-step-guide-to-make-code-changes/
https://about.gitlab.com/handbook/engineering/ux/ux-scorecards/
https://about.gitlab.com/handbook/engineering/ux/uxdesigner-onboarding/

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.