[Docs] Add reinforcement learning example illustrating gpu-to-gpu RDT and GRPO. (ray-project#57961)

## Description
Example for first blog in the RDT series using NIXL for GPU-GPU tensor
transfers.

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Signed-off-by: Ricardo Decal <public@ricardodecal.com>
Signed-off-by: Stephanie Wang <smwang@cs.washington.edu>
Co-authored-by: Ricardo Decal <public@ricardodecal.com>
Co-authored-by: Stephanie Wang <smwang@cs.washington.edu>
Co-authored-by: Qiaolin Yu <liin1211@outlook.com>

Author: 4 people