Send the Slack WebHook URL as a Secret

When setting up the action to send Slack notification, the Slack WebHook URL need to send as a plain text.
This URL should be sent as a secret value as it contains sensitive data.
Or else there should be a way to provide the whole config via a secret (_/templates/cfg-configmap.yaml_).

Start with _/templates/cfg-configmap.yaml_ and inspect how the Slack notification action currently receives its WebHook URL. Determine whether the configuration can consume a secret value or whether the whole config must be supplied from a secret; done means the URL is no longer required as plain text and the supported configuration path is documented or tested.

Written by the indexing model from the issue text.