### 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
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Signed-off-by: whx-sjtu <2952154980@qq.com>