Organize translated docs and refine README demo thumbnails

[PangMo5]

Keep the repository root focused on the English documents and make demo links consistent across the README feature sections.

Changes

• Move translated READMEs and notices to docs/<locale>/, and DemoLab translations to DemoLab/docs/<locale>/. Update language navigation, image paths, notice links, and workflow paths together.
• Add one 160px right-aligned thumbnail to each of eight feature sections. Each thumbnail links to its web demo; the introductory media retains its existing size.
• Remove the awkward “Auto-open demo” feature label and em dashes from current guidance.
• Replace Borrow posters in all five languages with frames showing the borrowed Chat workspace fully docked.
• Add explicit poster-time selection and use image hashes for poster URLs, so a poster-only change refreshes correctly without re-encoding the video.

Validation

• 23 Tools tests passed on both macOS and Linux, including poster selection, cache invalidation, and preview routing.
• Verified 65 generated documents, 108 localized scenes, and 20 site pages.
• Checked relocated document/image links and localized website notice destinations.
• GitHub Markdown rendering preserved the right alignment, 160px width, and section clearing. Inspected the rendered README and all five replacement Borrow posters.
• No MP4 files changed. Staged diff and secret checks passed.

Merging runs the existing site deployment workflow. No app version bump or release tag is included.

Linux CI: https://github.com/PangMo5/Tatami/actions/runs/34112030574