Add llms.txt generation script

[a2u]

Summary

• Add scripts/generate-llms-txt.js to generate LLM-friendly documentation files
• Generates llms.txt (index with links) and llms-full.txt (full content)
• Integrates into docs:build pipeline automatically
• Copies markdown files to dist with proper naming (README.md → index.md)

Made with Cursor

[yel-hadd]

Thanks for the PR — the main implementation is correct and works well.
I reviewed/build-tested it locally and added a small follow-up focused on output consistency and structure safety.

What was changed and why

1. Sidebar key normalization for Imunify Patch

   • Changed key in docs/.vuepress/config-client/sidebar.ts from:
     • '/imunify_patch' -> '/imunify_patch/'
   • Why: all other sidebar section keys use trailing slashes. Keeping this consistent avoids subtle route-key mismatches in generators/parsers and reduces future maintenance risk.

2. Exclude non-README markdown pages from llms generation

   • Updated scripts/generate-llms-txt.js so llms.txt / llms-full.txt are generated strictly from documented structure (documents.ts + sidebar.ts) instead of auto-including extra standalone *.md files.
   • Why: pages like docs/whmcs_plugin/whmcs_saved.md were being surfaced even though they are not part of intended VuePress navigation structure.

3. Dynamic title sanitization in generated llms output

   • In scripts/generate-llms-txt.js, title generation now converts inline HTML markup to plain text (natural-language-compatible structure).
   • This applies generally to HTML-decorated titles, and also preserves deprecation semantics by normalizing badge-style markup to [DEPRECATED].
   • Why: llms.txt should be clean/plain-text friendly for LLM/indexing consumers while preserving meaning.

4. WHMCS docs cleanup

   • Deleted docs/whmcs_plugin/whmcs_saved.md (not part of visible site structure).
   • Updated link in docs/whmcs_plugin/README.md to:
     • https://docs.cloudlinux.com/cln/whmcs_plugin/
   • Note: https://docs.cln.cloudlinux.com/whmcs_plugin/ is a redirect; this change points directly to the canonical destination.
   • Why: aligns docs references and avoids relying on redirect-only entry URLs.

Validation summary

• yarn docs:build passes.
• llms.txt and llms-full.txt are generated successfully.
• No whmcs_saved.md links remain in generated llms output.
• HTML-decorated titles are normalized to plain-text-compatible output.
• Generated URLs resolve locally.
• imunify_patch route was specifically regression-checked and remains non-breaking.

@annkots please review these follow-up adjustments.
Core PR behavior is mostly correct — these are targeted cleanup changes for consistency, output hygiene, and edge-case safety.