PerformanceMonitoring.md: add new page and remove PerfDashboard.md

This change removes the old and out-of-date PerfDashboard.md and
replaces it with documentation on our actual working performance
monitoring system.

Change-Id: I8aeb735bae5563cd4caf43523b0f6409b6a2d8e3
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/563495
Reviewed-by: David Chase <drchase@google.com>