[P/D] Add readme for PD separation (#4182)

### What this PR does / why we need it?
Add readme for PD separation

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
By ci

- vLLM version: v0.11.0
- vLLM main:
vllm-project/vllm@2918c1b

---------

Signed-off-by: wangxiaoteng <wangxiaoteng@huawei.com>
Signed-off-by: liziyu <liziyu16@huawei.com>
Co-authored-by: liziyu <liziyu16@huawei.com>

Author: wangxiaoteng888

docs/source/tutorials/index.md

@@ -20,7 +20,6 @@ DeepSeek-V3.2-Exp.md
 multi_node
 multi_node_kimi
 multi_node_qwen3vl
-multi_node_pd_disaggregation_llmdatadist
 multi_node_pd_disaggregation_mooncake
 multi_node_ray
 :::