[5/N][Attention] Finish eliminating vllm/attention folder (#32064)

Signed-off-by: Matthew Bonanni <mbonanni@redhat.com>

docs/contributing/model/basic.md

@@ -29,7 +29,7 @@ The initialization code should look like this:
     ```python
     from torch import nn
     from vllm.config import VllmConfig
-    from vllm.attention.layer import Attention
+    from vllm.model_executor.layers.attention import Attention
 
     class MyAttention(nn.Module):
         def __init__(self, vllm_config: VllmConfig, prefix: str):

docs/design/custom_op.md

@@ -271,7 +271,7 @@ Taking `MMEncoderAttention` as an example:
 ??? code
 
     ```python
-    from vllm.attention.layers.mm_encoder_attention import MMEncoderAttention
+    from vllm.model_executor.layers.attention import MMEncoderAttention
     from vllm.model_executor.custom_op import CustomOp