[Doc] Add developer guide for CustomOp (#30886)

Signed-off-by: shen-shanshan <467638484@qq.com>

vllm/model_executor/models/transformers/moe.py

@@ -37,10 +37,13 @@ if TYPE_CHECKING:
     from vllm.config import VllmConfig
 
 
+# --8<-- [start:transformers_fused_moe]
 @CustomOp.register("transformers_fused_moe")
 class TransformersFusedMoE(FusedMoE):
     """Custom FusedMoE for the Transformers modeling backend."""
 
+    # --8<-- [end:transformers_fused_moe]
+
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
         self._topk_ids: torch.Tensor = None