ICRC-29: broadcast ready message

[peterpeterparker]

Motivation

The icrc29_ready cannot be scoped to a particular origin because the origin is unknown at the time the notification is emitted. Therefore, I think it is worth highlighting in the specification that this is a broadcast.

Notes

Limitation, or approach, is notably similar to Internet Identity specification for authorize-ready.

[peterpeterparker]

Discussed offline. I understood this specification as the specification of the "ready" message while it actually covers the general data exchange flow. There I close this PR and let @frederikrothenberger provides adjustments to the structure of the document if necessary in another PR.