docs(architecture): add bilingual DeepEP deep-dive

[tianhao909]

Summary

• add a source-driven DeepEP documentation tree under docs/ with both English and Chinese versions
• cover architecture, quick start, normal kernels, low-latency kernels, math intuition, and performance tuning
• link the repository README to the new bilingual documentation index

Validation

• bash ./format.sh
• python tests/test_intranode.py (requires an NVLink-capable multi-GPU machine)
• python tests/test_internode.py (requires an NVLink + RDMA + NVSHMEM multi-node cluster)
• python tests/test_low_latency.py (requires RDMA + NVSHMEM and the intended serving topology)

Notes

• The repository does not include a separate CONTRIBUTING.md or PR template, so this change follows the existing README guidance and the format workflow in .github/workflows/format.yml.
• The runtime tests are hardware-gated and cannot be honestly exercised in a generic environment without the required topology.