### What this PR does / why we need it?
This PR adds a load-balance dp proxy server which can be used in
external DP scenario without Disaggregated-Prefill enabled. What's more,
add a doc of external dp and load-balance dp proxy server.
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
See the new doc.
- vLLM version: v0.11.0
- vLLM main:
2918c1b49c
---------
Signed-off-by: whx-sjtu <2952154980@qq.com>
This PR re-implements external-dp starter based on vllm's support for
external dp.
- vLLM version: v0.10.1.1
- vLLM main:
f38035c123
---------
Signed-off-by: whx-sjtu <2952154980@qq.com>
