Extend alerting documentation with information about using Kibana keystore for preconfigured connectors

[YulNaumenko]

Currently, we have a documentation for preconfigured connectors definition, where is mentioned that sensitive properties, such as passwords, should be stored in the Kibana keystore. But it's not clear how to actually do that. A pointer to existing documentation is probably good enough - there's this - https://www.elastic.co/guide/en/kibana/current/secure-settings.html - but it still doesn't explain how I'd use it.

For the slack example, you would set this key in the keystore: xpack.actions.preconfigured.my-slack1.config.webhookUrl, and then leave off the config property in the kibana.yml for that preconfigured action.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)