Begin splitting Prometheus page

Problem to solve

To hit the 13.0 release target, we had to merge the work from !31933 (merged) into the already-overloaded https://docs.gitlab.com/ee/user/project/integrations/prometheus.html page in the spirit of MVC. (Current size: 52K) Now we need to clean up the technical debt we've created, and start moving some content out of that page for real.

We want to strike a careful balance here; @nadia_sotnikova and others in UX are working on a card-sorting exercise that will be a more complete version of what's being done here, but there are a few items we can start moving to take some of the weight off of the Prometheus page. From conversations in !31933 (merged) and in Slack, they are:

1. In /user/project/ create a new subfolder: /user/project/metrics
2. Topics like "Custom metrics" and "Metric embeds" can go in that new /user/project/metrics/index.md` for now.
3. In the new /user/project/metrics folder, also create a new subfolder: /user/project/metrics/dashboards
4. Carve off at least some portion of the Prometheus page (exact portions to be determined)

cc @jivanvl @rpereira2 @ClemMakesApps

Further details

Proposal

Who can address the issue

Other links/references

Related to #227441 (closed) where the work truly happened.