[Platform] Enable ARM-only CPU binding with NUMA-balanced A3 policy and update docs/tests (#6686)

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

- Keeps enable_cpu_binding default on, but skips binding on non‑ARM CPUs
inside bind_cpus, with a clear log.
- Uses a table-driven binding policy: A3 uses NUMA‑balanced binding;
other device types use NUMA‑affinity binding.
- Updates docs to reflect the exact behavior and adds/updates unit tests
for the new logic.

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

- Yes. CPU binding is now enabled by default via additional_config, and
documented in the user guide.
- CPU binding behavior differs by device type (A3 vs. others).

### How was this patch tested?

Added/updated unit tests:

test_cpu_binding.py
1.   test_binding_mode_table covers A2 vs A3 binding mode mapping.
2. test_build_cpu_pools_fallback_to_numa_balanced covers fallback when
affinity info is missing.
3. TestBindingSwitch.test_is_arm_cpu covers ARM/x86/unknown arch
detection.
4.   test_bind_cpus_skip_non_arm covers non‑ARM skip path in bind_cpus.

test_worker_v1.py
1. Updated mocks for enable_cpu_binding default True to align with new
config default.

- vLLM version: v0.14.1
- vLLM main: d7de043

---------

Signed-off-by: chenchuw886 <chenchuw@huawei.com>
Co-authored-by: chenchuw886 <chenchuw@huawei.com>

vllm_ascend/ascend_config.py

@@ -84,7 +84,7 @@ class AscendConfig:
         self.multistream_overlap_shared_expert = additional_config.get("multistream_overlap_shared_expert", False)
         self.multistream_overlap_gate = additional_config.get("multistream_overlap_gate", False)
         self.recompute_scheduler_enable = additional_config.get("recompute_scheduler_enable", False)
-        self.enable_cpu_binding = additional_config.get("enable_cpu_binding", False)
+        self.enable_cpu_binding = additional_config.get("enable_cpu_binding", True)
 
         self.pd_tp_ratio = 1
         self.pd_head_ratio = 1