docs: add new GPU acceleration guide for spatial joins

[pwrliang]

What this PR does

This PR adds a new documentation page at docs/gpu-acceleration.ipynb and docs/gpu-acceleration.md that introduces GPU-accelerated spatial joins in SedonaDB.

Highlights

• Adds prerequisites for GPU usage (CUDA/version, compute capability, RT core notes).
• Documents GPU join execution model (filtering + refinement) and supported predicates.
• Explains fallback behavior with gpu.fallback_to_cpu.
• Provides build-from-source guidance for GPU (MATURIN_PEP517_ARGS="--features gpu"), including related environment variables.
• Adds runtime configuration examples for gpu.* options and batch-size tuning.
• Includes a complete example workflow:
  • check GPU availability (nvidia-smi)
  • download sample data
  • create external tables
  • run CPU vs GPU benchmark in notebook-style code
  • display timing/speedup output

Why

Users need a practical, end-to-end guide to enable, tune, and validate GPU spatial join performance.

Scope

• Documentation only
• New file: docs/gpu-acceleration.md
• No runtime/engine behavior changes

Notes

Benchmark numbers shown in the example are environment-dependent and should be treated as illustrative.

[zhangfengcdt]

@pwrliang Could you please fix the missing license header error in ci? Thanks!

[pwrliang]

@pwrliang Could you please fix the missing license header error in ci? Thanks!

Fixed, thank you.