build_queued: document in the notifications docs

Dependencies

• [2] 6A5QEB4Q Write up some documentation on notifications

Change contents

• edit in doc/manual/src/notifications.md at line 10
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  + ### `build_queued`

  +

  + * **Payload:** Exactly one value, the ID of the build.

  + * **When:** Issued after the transaction inserting the build in to the database is committed. One notification is sent per new build.

  + * **Delivery Semantics:** Ephemeral. `hydra-notify` must be running to react to this event. No record of this event is stored.