[Example] Add paged block-sparse flash-decoding kernel (#638)

* Add paged block-sparse flash-decoding kernel

* Update example_tilelang_sparse_gqa_decode_paged.py

* lint fix

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Co-authored-by: Lei Wang <34334180+LeiWang1999@users.noreply.github.com>
Co-authored-by: LeiWang1999 <leiwang1999@outlook.com>

examples/blocksparse_attention/README.md

+# Block-Sparse Flash-Attention
+
+Tilelang implementation of block-sparse flash-attention kernels. 
+
+The kernels have been used in [Rectified Sparse Attention](https://arxiv.org/abs/2506.04108) and [SeerAttention-R](https://arxiv.org/abs/2506.08889). 
+