docs: update vLLM loader registration docs for 0.4.0

[zhengluo-nv]

Summary

• Backport docs: update vLLM loader registration docs #390 onto release/0.4.0.
• Document --load-format modelexpress as the canonical vLLM load format and mx as the backward-compatible alias.
• Remove stale mx-source / mx-target references and obsolete --worker-cls quick-start usage.
• Remove unsupported MX_REGISTER_LOADERS docs and point users to vLLM plugin registration, VLLM_PLUGINS=modelexpress when explicit plugin selection is needed, or manual register_modelexpress_loaders().

Bugs

• NVBug 6235739
• NVBug 6235844

Validation

• git diff origin/release/0.4.0 --check
• rg -n 'MX_REGISTER_LOADERS' . returns no matches
• stale docs search for --load-format mx, mx-source / mx-target, and --worker-cls returns no matches in the docs/example scope
• python3 -m py_compile modelexpress_client/python/modelexpress/vllm_worker.py
• Parsed touched YAML manifests with Ruby YAML