Resolve "Update LCP metric to reference data source"

Why is this change being made?

Last effort to complete this issue: https://gitlab.com/gitlab-data/analytics/-/issues/7070

Add context to LCP metric to inform readers where the data is coming from.

We want to point the data to this page: https://gitlab.com/gitlab-data/analytics/tree/master/extract/graphite

Review link: https://10202-update-lcp-metric-to-reference-data-source.about.gitlab-review.app/handbook/engineering/development/performance-indicators/#largest-contentful-paint-lcp

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 #10202 (closed)