[DOCS] Add alert source to detection rule action context

[jmikell821]

Issue: elastic/kibana#85488

Docs update: Alert data is now available in detection rule actions at {{context.alerts}} as an array. This array contains each alert generated since the last time the action executed. Mustache templating can be used to iterate over all alerts in the array and capture information from each one. For example, {{#context.alerts}}Detection alert for user: {{user.name}}{{/context.alerts}} would create the string Detection alert for user: <user.name> for every alert in the array. Any alerts that don't have user.name will still generate the string but leave <user.name> blank.

[jmikell821]

Merged #450.