[Website]: Document ECH built-in SMTP connector's FROM domain requirement for xpack.actions.email.domain_allowlist

### Before you submit

- [x] This issue is about a documentation page, flow, or piece of content.

### Type of issue

Missing information

### What documentation page or section is affected

https://www.elastic.co/docs/reference/kibana/configuration-reference/alerting-settings

### What happened?

The `xpack.actions.email.domain_allowlist` setting will cause Kibana to restrict the FROM address.

>xpack.actions.email.domain_allowlist
>A list of allowed email domains which can be used with the email connector. When this setting is not used, all email domains are allowed. When this setting is used, if any email is attempted to be sent that (a) includes an addressee with an email domain that is not in the allowlist, or (b) includes a from address domain that is not in the allowlist, it will fail with a message indicating the email is not allowed.

On ECH, the built-in SMTP connector's `FROM` address is likely unknown to the Kibana user ahead of time. So if the `domain_allowlist` setting is applied, they will likely encounter a validation error (screenshot below). At which point they would need to apply the domain from the error into the `domain_allowlist`.

Consider updating the setting description to note this scenario and the domains needed for alerting per region and/or recommending the use of `recipient_allowlist` instead if no custom email connectors will be used.

### Additional info

[This email action page](https://www.elastic.co/docs/reference/kibana/connectors-kibana/email-action-type) might be worth updating as well
>When the [xpack.actions.email.domain_allowlist](https://www.elastic.co/docs/reference/kibana/configuration-reference/alerting-settings#action-config-email-domain-allowlist) configuration setting is used, the email addresses used for all of the Sender (from), To, CC, and BCC properties must have email domains specified in the configuration setting.

Review the alerting settings page at https://www.elastic.co/docs/reference/kibana/configuration-reference/alerting-settings, especially xpack.actions.email.domain_allowlist, and compare the email action type page linked in the issue. Document that ECH's built-in SMTP connector may use an unknown FROM domain, explain the required allowlist behavior and regional domains if confirmed, and mention recipient_allowlist where appropriate.

Written by the indexing model from the issue text.