docs(console): document the Email destination of notification rules

[ziracmo]

Notification rules can now send their notifications by email, to members of the organization.

• Notification rule: new Email destination section (recipients picked among members, 1 to 10 per destination, events not sent by email and why). The prerequisite note no longer says an integration must be linked first, since Email needs none.
• Overview: Email listed among the available integrations, with no linking step.

[aws-amplify-eu-west-1]

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