docs: rewrite the README for a public audience

[xAlcahest]

Summary

The README opened with history and a feature list; it now opens with what the app does for a rider and why the implementation is worth reading. The technical sections (Docker, refresh, sync, smoke test, architecture) are unchanged apart from a new Languages section and a paragraph on vehicle motion between polls.

Changes

• README.md: new opening and "What it does" section, Deploying summary, Languages section, motion paragraph in the stop map section.

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