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chore(docs): add server and client side changes to KEP-2839

The KEP was transferred from kubeflow/sdk#285 scoped SDK-only, but landing TRL
needs a runtime image and manifests here too.

Split into Part I (server) and Part II (client). Part I is new: the launcher
adapter contract, the trl-launch adapter and the runtime manifests. The control
plane is untouched -- no CRD change and no Go, since every configuration worth
rejecting is a property of the image's launcher.

A PoC on two nodes found accelerate ignores PET_*: correct flag translation
still gave world_size 1 with both pods Succeeded, while a torchrun probe on the
same nodes reached world_size 2. Hence the adapter is specified here rather
than left to image authors.

Also moves the file to proposals/, following the directory move upstream, and
condenses the inherited SDK sections: repeated class listings become tables and
the eight alternatives become one, keeping every decision and reference.