feat(docs): add English llms output

[loicduong]

Summary

Fixes #34

• Add vitepress-plugin-llms to the docs build
• Configure the plugin to generate LLM-friendly output from docs/en
• Update npm lockfile for the new VitePress plugin dependency

Verification

• npm run docs:build

[loicduong]

@sanbuphy I've updated the llms.txt configuration, please review it.