Add First Session quickstart guide and cross-links

[bseverns]

Motivation

• Provide an immediate, teachable 10‑minute onboarding ritual so instructors can verify the full hardware → firmware → serial → OSC → visualizer pipeline.
• Make it trivial to recover when you only have a short setup window by surfacing the fastest end‑to‑end steps from the bridge doc.
• Offer a minimal, deterministic test packet and expected visual response so users can validate the system quickly.
• Correct the p5 visualizer path reference so the browser sketch is discoverable.

Description

• Add docs/FirstSession.md containing a step‑by‑step quickstart, the known‑good packet {"gesture":"pluck","value":96}, and the expected visual response (PLUCK and velocity ≈ 96/127).
• Insert a First Session heading in README.md linking to docs/FirstSession.md and summarizing the 10‑minute script.
• Add an If you only have 10 minutes pointer to docs/OSCSerialBridge.md that directs readers to docs/FirstSession.md.
• Fix the p5 visualizer reference in the new doc to software/p5js/sketch.js.

Testing

• No automated tests were executed because this is a documentation‑only change.

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Codex Task