Added monitoring docs

doc/development/new_fe_guide/development/performance.md

 # Performance
 
-> TODO: Add content
+## Monitoring
+
+We have a performance dashboard available in one of our [grafana instances](https://performance.gprd.gitlab.com/dashboard/db/sitespeed-page-summary?orgId=1). This dashboard automatically aggregates metric data from [sitespeed.io](https://sitespeed.io) every 6 hours. These changes are displayed after a set number of pages are aggregated.
+
+These pages can be found inside a text file in the gitlab-build-images [repository](https://gitlab.com/gitlab-org/gitlab-build-images) called [gitlab.txt](https://gitlab.com/gitlab-org/gitlab-build-images/blob/master/scripts/gitlab.txt)
+Any frontend engineer can contribute to this dashboard. They can contribute by adding or removing urls of pages from this text file. Please have a [frontend monitoring expert](https://about.gitlab.com/team) review your changes before assigning to a maintainer of the `gitlab-build-images` project. The changes will go live on the next scheduled run after the changes are merged into `master`.
+
+There are 3 recommended high impact metrics to review on each page
+
+* [First visual change](https://developers.google.com/web/tools/lighthouse/audits/first-meaningful-paint)
+* [Speed Index](https://sites.google.com/a/webpagetest.org/docs/using-webpagetest/metrics/speed-index)
+* [Visual Complete 95%](https://sites.google.com/a/webpagetest.org/docs/using-webpagetest/metrics/speed-index)
+
+For these metrics, lower numbers are better as it means that the website is more performant.