docs: add UST.space ratings extension guide

[MisterCommand]

Summary

• add a Timetable Planner guide for downloading and loading the packaged UST.space Ratings Chrome extension
• warn that the extension has no official support and must be used at the user's own risk
• include an explicit placeholder for the future extension ZIP download URL
• describe rating badges, automatic retrieval, account-backed saving and cross-device synchronization, seven-day search refreshes, filters, permissions, and troubleshooting
• add MDX-compatible placeholders for setup, ratings, filter, and access-warning screenshots
• link the guide from the Timetable Planner overview

Verification

• checked the documentation against the extension manifest, source, and planner integration
• confirmed that the public setup contains no repository clone, Bun, or source-build instructions
• verified the branch diff contains only the new guide and overview link
• Docusaurus build not run because the docs repository was edited through the GitHub connector

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