[Feat][Doc] Add a load_balance_dp_proxy in examples and external dp doc. (#4265)

### 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>

examples/external_online_dp/run_dp_template.sh

@@ -2,7 +2,6 @@ export HCCL_IF_IP=your_ip_here
 export GLOO_SOCKET_IFNAME=your_socket_ifname_here
 export TP_SOCKET_IFNAME=your_socket_ifname_here
 export HCCL_SOCKET_IFNAME=your_socket_ifname_here
-export DISAGGREGATED_PREFILL_RANK_TABLE_PATH=your_rank_table_path_here
 export VLLM_LOGGING_LEVEL="info"
 export OMP_PROC_BIND=false
 export OMP_NUM_THREADS=10
@@ -24,21 +23,10 @@ vllm serve model_path \
     --enable-expert-parallel \
     --seed 1024 \
     --served-model-name dsv3 \
-    --max-model-len 3500 \
-    --max-num-batched-tokens 3500 \
-    --max-num-seqs 28 \
+    --max-model-len 8192 \
+    --max-num-batched-tokens 2048 \
+    --max-num-seqs 16 \
     --trust-remote-code \
     --gpu-memory-utilization 0.9 \
     --quantization ascend \
-    --speculative-config '{"num_speculative_tokens": 1, "method":"deepseek_mtp"}' \
-    --kv-transfer-config \
-    '{"kv_connector": "LLMDataDistCMgrConnector",
-      "kv_buffer_device": "npu",
-      "kv_role": "kv_consumer",
-      "kv_parallel_size": "1",
-      "kv_port": "20001",
-      "engine_id": "0",
-      "kv_connector_module_path": "vllm_ascend.distributed.llmdatadist_c_mgr_connector"
-    }' \
-    --additional-config \
-    '{"ascend_scheduler_config": {"enabled": true}, "torchair_graph_config":{"enabled":true,"enable_kv_nz":false, "graph_batch_size":[28]}, "enable_weight_nz_layout":true, "enable_multistream_moe":false}'
+    --speculative-config '{"num_speculative_tokens": 1, "method":"deepseek_mtp"}' \