Highlight RTX 50 CUDA 12.8 quick start

[peter941221]

This makes the top-level README surface the existing RTX 50 series Docker guidance earlier.

Failure mechanism:

• doc/en/docker_use.md and doc/en/docker_build.md already point RTX 50 users to CUDA 12.8
• the top-level README quick start still centered the CUDA 12.6 image with no explicit RTX 50 fast path
• that makes first-run setup easy to choose incorrectly on Blackwell consumer GPUs

Semantic change:

• add an explicit RTX 50 / CUDA 12.8 recommendation before the Docker quick start block
• add matching 12.8 quick-start commands in both README.md and README_ZH.md

Preserved invariant:

• existing CUDA 12.6 instructions remain available for the broader GPU set they already target
• no install or runtime behavior changes

Testing:

• compared the new README wording against doc/en/docker_use.md and doc/en/docker_build.md
• mirrored the same guidance in README_ZH.md