Update docs to reflect recommended method for integrating with pebble log forwarding

[sed-i]

Enhancement Proposal

Charmers will soon need to update their pebble layers with the loki push api. Our docs should be ready for this.

Related: https://github.com/canonical/postgresql-k8s-operator/pull/486

[lucabello]

We have docs for Pebble Log Forwarding now :)

https://charmhub.io/loki-k8s/docs/pebble-log-forwarding

Should we add some info about adding a Pebble service that tails a log file?