Create a `metrics` page on the site

[deemp]

We need to refer to the documentation about metrics on some site pages and in the report.

TODO

• [ ] Create a page Metrics on the site. The page should explain
  • [ ] the metrics that we use
  • [ ] why do we use these metrics
  • [ ] what improvements (relative changes w.r.t initial metrics) we want to achieve
  • [ ] where users can see reports
• [ ] Move the documentation about metrics from the report to the site. The report is generated by the normalizer (link).
• [ ] Add links on the site where necessary
  • [ ] pipeline.md