docs: add CUDA operator practice resource

[kkcocoa]

What changed

• Added the CUDA operator interview-practice site to the CUDA programming and operator-optimization resource list.

Why

This interactive practice resource complements the existing CUDA tutorials with hands-on drills for memory access, Reduce, Softmax, Attention, and GEMM.

Validation

• Ran astro check and astro build successfully (0 errors).
• Confirmed the resource link and Markdown table formatting.