[Bugfix] Fix intermittent kv_port conflict with AscendDirectTransport (#6455)

### What this PR does / why we need it?

When using Mooncake on Ascend NPU, AscendDirectTransport randomly
allocates ports within range `[20000, 20000 + npu_per_node × 1000)`.
Reference:
[ascend_direct_transport.cpp#L554](https://github.com/kvcache-ai/Mooncake/blob/v0.3.7.post2/mooncake-transfer-engine/src/transport/ascend_transport/ascend_direct_transport/ascend_direct_transport.cpp#L475)

If `kv_port` overlaps with this range, users may encounter intermittent
startup failures:
```bash
zmq.error.ZMQError: Address already in use (addr='tcp://x.x.x.x:30012')
RuntimeError: KV Cache sending/receiving thread failed to start.
```
This pr fix intermittent kv_port conflict with AscendDirectTransport in
`Qwen3-235B-W8A8-EPLB.yaml`, and add Added `kv_port Configuration Guide`
section in `pd_disaggregation_mooncake_multi_node.md`.

test
Results(tests/e2e/nightly/multi_node/config/Qwen3-235B-W8A8-EPLB.yaml):
https://github.com/vllm-project/vllm-ascend/actions/runs/21540138907/job/62073265259

### Does this PR introduce _any_ user-facing change?

### How was this patch tested?

- vLLM version: v0.14.1
- vLLM main:
dc917cceb8

Signed-off-by: Meihan-chen <jcccx.cmh@gmail.com>

tests/e2e/nightly/multi_node/config/Qwen3-235B-W8A8-EPLB.yaml

@@ -37,7 +37,7 @@ deployment:
         --kv-transfer-config
         '{"kv_connector": "MooncakeConnectorV1",
         "kv_role": "kv_producer",
-        "kv_port": "30000",
+        "kv_port": "36000",
         "engine_id": "0",
         "kv_connector_extra_config": {
                   "prefill": {
@@ -73,7 +73,7 @@ deployment:
         --kv-transfer-config
         '{"kv_connector": "MooncakeConnectorV1",
         "kv_role": "kv_consumer",
-        "kv_port": "30200",
+        "kv_port": "36100",
         "engine_id": "1",
         "kv_connector_extra_config": {
                   "prefill": {