docs: Add NVFP4 implementation guide

Comprehensive technical guide covering the E2M1 format, two-level
micro-block scaling, Blackwell hardware (tcgen05/mma.sync), rotation-based
quantization, QuTLASS/FP-Quant/Four Over Six implementations, and
CUDA-level implementation details for NVFP4 in bitsandbytes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: TimDettmers