Document how to clean out old/stale Graphite data

[lunkwill42]

Is your feature request related to a problem? Please describe.

When a device or component is removed from NAV, NAV will (of course) stop dispatching metrics to Graphite for this component.

Graphite never removes metrics, and (as far as we know) has no API to remove obsolete metrics. These will remain on-disk "forever", despite never being used again.

Describe the solution you'd like

Internally, Sikt uses a cronjob to automatically deletes Whisper files from a Graphite server when they have not been updated in over a year. It has been suggested by the NAV reference committee that this cron job be included either as a source code contrib or as a documented (Sphinx docs) example of how to do it.