Document the Notifications send API

[eirsep]

Description

Documents the Notifications send API, POST /_plugins/_notifications/feature/send, which sends a message to existing channels: request fields, example request and response, and the permissions it requires. Adds the notifications_send_access predefined role to the Security plugin's roles table.

Issues Resolved

Related: opensearch-project/notifications#1279, opensearch-project/notifications#1280, opensearch-project/security#6602

Version

3.10

Frontend features

N/A

Checklist

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