Use one documentation theme for CUDA 12 and CUDA 13 bindings #3403

— codex GPT-6 on behalf of Ralf —

Follow-up to Mike's review, for the post-migration documentation deduplication work.

CUDA 12 bindings docs currently use Furo, while CUDA 13 uses the NVIDIA Sphinx theme.

Use one theme for both roots and share common configuration where practical. Preserve version selection, preview banners, and published URLs, and validate both documentation builds.