[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>

tests/ut/worker/test_worker_v1.py

@@ -70,7 +70,7 @@ class TestNPUWorker(TestBase):
         # Setup mock behavior
         mock_ops.register_dummy_fusion_op.return_value = None
         mock_ascend_config = MagicMock()
-        mock_ascend_config.enable_cpu_binding = False
+        mock_ascend_config.enable_cpu_binding = True
         mock_get_ascend_config.return_value = mock_ascend_config
 
         # Import and create NPUWorker instance
@@ -125,7 +125,7 @@ class TestNPUWorker(TestBase):
         self.model_config_mock.trust_remote_code = True
         mock_ops.register_dummy_fusion_op.return_value = None
         mock_ascend_config = MagicMock()
-        mock_ascend_config.enable_cpu_binding = False
+        mock_ascend_config.enable_cpu_binding = True
         mock_get_ascend_config.return_value = mock_ascend_config
 
         # Create NPUWorker instance
@@ -168,7 +168,7 @@ class TestNPUWorker(TestBase):
         self.cache_config_mock.cache_dtype = "float32"
         mock_ops.register_dummy_fusion_op.return_value = None
         mock_ascend_config = MagicMock()
-        mock_ascend_config.enable_cpu_binding = False
+        mock_ascend_config.enable_cpu_binding = True
         mock_get_ascend_config.return_value = mock_ascend_config
 
         # Create NPUWorker instance