[Doc] Document ECMooncakeConnector for EPD disaggregation

[stmatengss]

Purpose

Document ECMooncakeConnector for Encode-Prefill-Decode (encoder-cache) transfer over Mooncake Transfer Engine.

vLLM merged the connector in vllm-project#41567, but docs/features/disagg_encoder.md still described filesystem ECExampleConnector as the only path. This adds a usage guide and links it from the disaggregated encoder, prefill, MooncakeConnector, and example README pages.

Why this is not duplicating an existing PR

• Open search for ECMooncake / EPD docs on vllm-project/vllm found no documentation PR covering this connector.
• [EC Connector] Mooncake EC Connector for Distributed Encoder-Cache Transfer vllm-project/vllm#40695 is an older connector implementation, not docs.
• Mooncake-side docs for the same EPD path were prepared separately; this PR is the vLLM user guide.

Test Plan

Docs-only. Commands run:

# Confirm connector public API used in the guide still matches main
rg -n "mooncake_protocol|ec_ip|ec_port|ec-consumer-zmq-addrs" \
  vllm/distributed/ec_transfer/ec_connector/mooncake/config.py \
  examples/disaggregated/disaggregated_encoder/disagg_epd_proxy.py

Test Result

• Guide matches MooncakeECConfig (mooncake_protocol default rdma, consumer ec_ip/ec_port) and proxy --ec-consumer-zmq-addrs (E+PD only).
• No model-serving eval; this change does not affect output, accuracy, or serving.

AI assistance

AI assistance was used to draft the guide from the merged connector, proxy, and run_epd_mooncake_ec_full_pipeline.sh. A human submitter should review every changed line before marking ready.

Checklist

• Purpose of the PR
• Test plan
• Test results
• Documentation update (this PR)

  

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