Resolve "Add chart guideline to metrics page"

Why is this change being made?

Engineering Performance Indicators Page: https://about.gitlab.com/handbook/engineering/performance-indicators/#guidelines

• We should add a link to the Engineering Metrics page.
• Review link: https://11634-add-chart-guideline-to-metrics-page.about.gitlab-review.app/handbook/engineering/performance-indicators/index.html#guidelines

Engineering Metrics Page: https://about.gitlab.com/handbook/engineering/metrics/

• We should have a section for guidelines on our engineering metrics page.
• Review link: https://11634-add-chart-guideline-to-metrics-page.about.gitlab-review.app/handbook/engineering/metrics/index.html#guidelines

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)

Closes #11634 (closed)