docs: add llms.txt index

[kyui-azusa]

Summary

• add a root-level llms.txt for language-model-friendly documentation discovery
• link the main guides, script reference, publishing docs, localized docs, and source repositories
• include concise WebGAL script conventions for consumers

Validation

• yarn build
• verified src/.vuepress/dist/llms.txt matches the source file

Closes #82