Move CPU-usage design under docs/Features/Admin-Panel/Problems/ and add
RAM-usage and Disk-usage design docs (siblings of CPU-usage): both log
how much RAM+swap / disk is used and record only the START and END of
each sustained high-usage period to Admin Panel / Problems, mirroring the
CPU-usage monitor's shape (hysteresis tracker, event stream, env vars).
Link them from the Admin-Panel index and update the now-automatic card
loading description. Update the CHANGELOG link to the moved CPU-usage.
