[PD Disaggregation]Set adxl engine as default backend and update README (#3761)

### What this PR does / why we need it?
Set adxl engine as the default Mooncake backend, because Ascend
Transport is no longer maintained.
Update README to include instructions for installing the adxl backend
Mooncake.
### Does this PR introduce _any_ user-facing change?
Users need to compile and install the mooncake backend for adxl
according to the revised README instructions.
### How was this patch tested?
By CI.

- vLLM version: v0.11.0
- vLLM main:
83f478bb19

Signed-off-by: nwpu-zxr <zhouxuerong2@huawei.com>

vllm_ascend/distributed/mooncake_connector.py

@@ -905,7 +905,7 @@ class MooncakeConnectorWorker:
         self.device_id = device_ids[self.tp_rank]  # type: ignore
 
         if vllm_config.kv_transfer_config.get_from_extra_config(
-                'use_ascend_direct', False):
+                'use_ascend_direct', True):
             hostname = self.side_channel_host
         else:
             hostname = f"{self.side_channel_host}:0:npu_{self.device_id}"

vllm_ascend/distributed/mooncake_layerwise_connector.py

@@ -662,7 +662,7 @@ class MooncakeLayerwiseConnectorWorker:
         self.device_id = device_ids[self.tp_rank]  # type: ignore
 
         if vllm_config.kv_transfer_config.get_from_extra_config(
-                'use_ascend_direct', False):
+                'use_ascend_direct', True):
             hostname = self.side_channel_host
         else:
             hostname = f"{self.side_channel_host}:0:npu_{self.device_id}"