docs: remove stale delta NCCL recommendation

[liuhao-labs]

Summary

• Remove the obsolete delta+NCCL recommendation from the English and Chinese external rollout engine decision tables.
• State the current contract explicitly: delta mode supports disk transport only; NCCL weight sync uses full mode.

The distributed delta+NCCL updater introduced by #1806 was removed by #2089. Current argument validation rejects --update-weight-mode delta --update-weight-transport nccl, but these two roadmap entries still recommended that exact combination. This aligns the documentation with the current runtime instead of suggesting a path that no longer exists.

This documentation update does not claim to repair the historical grouped-MoE corruption path reported in #2209; that code was removed from current main.

Tests

• English Sphinx build passed; only 50 pre-existing repository warnings, none from the changed page.
• Chinese Sphinx build passed; only 55 pre-existing repository warnings, none from the changed page.
• Verified generated HTML contains the new disk-only guidance.
• Pre-commit and git show --check passed.

Related to #2209