Improve documentation of kubernetes-config extension

[geoand]

Description

The current documentation of the quarkus-kubernetes-config is extremely bare-bones.
Having a typical example where some database credentials are obtained from a Secret would be very helpful.
Furthermore, comparing it to the alternative of using the Kubernetes extension to populate the Deployment with the proper entries could also be useful.

Implementation ideas

No response

[quarkus-bot]

/cc @iocanel