docs: improve generated llms.txt summaries

[ScriptedAlchemy]

Summary

Generated llms.txt descriptions for the Formatting and rs doc pages currently flatten body content into malformed or incomplete summaries. This PR adds explicit, aligned English and Chinese frontmatter descriptions so the generated indexes stay concise and readable.

Validation

• Built the native binding and checked formatting for the four changed MDX files.
• Ran cspell against only the four changed source files; ignored generated documentation outputs were intentionally excluded.
• Ran the full documentation build and release preparation, then verified the corrected English, Chinese, and packaged llms.txt entries and the absence of the malformed fragments.