docs: modernize README quick start

[gouyt13]

Summary

• reorganize the README around Python and C++ quick starts
• add a self-contained Python IVF example and clarify SIMD requirements
• add project badges, documentation links, SymphonyQG paper, citation, and license sections
• declare NumPy as a runtime dependency

Verification

• pytest --tb=short -q (54 passed)
• clean pip install built and installed rabitqlib with NumPy
• README Python IVF example returned the documented output shapes
• fresh C++ Release build succeeded
• ctest completed with 16/16 tests passing
• git diff --check