[Internal]: [ResponseOps] Provide an allow list for email recipient

[tiamliu]

Description

What: Adding a configuration xpack.actions.email.recipient_allowlist to white list email /domains to receive emails.
When: 9.2
Why: improved security and more granular configuration for email recipients

Resources

GH: elastic/kibana#210355
PR: elastic/kibana#220058

Which documentation set does this change impact?

Elastic On-Prem and Cloud (all)

Feature differences

identical in all deployments

What release is this request related to?

9.2

Serverless release

9.2

Collaboration model

The documentation team

Point of contact.

Main contact: @jcger

Stakeholders: @tiamliu

[nastasha-solomon]

@jcger was awesome and doc'd this already. Docs are published at https://www.elastic.co/docs/reference/kibana/configuration-reference/alerting-settings