[Alerting] Documentation for how to pre-configure connectors

[YulNaumenko]

We'll need documentation for how to pre-configure connectors. Place for a document: https://github.com/elastic/kibana/blob/master/docs/user/alerting/action-types.asciidoc

Once those are published (ex here: https://www.elastic.co/guide/en/kibana/master/action-types.html), we can use the doc links plugin to generate a URL.

[elasticmachine]

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