docs: split diffusion feature guides

Signed-off-by: Hongsheng Liu <liuhongsheng4@huawei.com>

docs/design/feature/skip_softmax.md

@@ -2,8 +2,10 @@
 
 Skip-Softmax is the sparse-attention mode of the `TRTLLM_ATTN` backend. Usage — the config keys
 and how to pick an operating point — is in
-[Diffusion Attention Backends](../../user_guide/diffusion/attention_backends.md). This page explains
-the algorithm.
+[TRTLLM Attention](../../user_guide/diffusion/attention_backends/trtllm.md#skip-softmax).
+The shared selector contract is documented in
+[Diffusion Attention Backend Selection](attention_backend_selection.md). This
+page explains the algorithm.
 
 ## Motivation