Add short accelerated tutorial videos (install / inference / training)

[GeorgePearse]

Summary

Add short, slightly-accelerated screen-recorded videos demonstrating the core VisDet workflows:

• Installation (emphasis: no repo clone required; uv pip install visdet)
• Inference (e.g. DetInferencer with a preset)
• Training (SimpleRunner and/or tools/train.py repo workflow)

Deliverables

• Install video (30–90s)
• Inference video (30–90s)
• Training video (60–120s)

Notes / Guidance

• Prefer terminal recordings, slightly accelerated (e.g. 1.25–1.5×)
• Show exact commands used (copy/paste friendly)
• Keep to minimal narration or none; captions optional
• Ensure outputs are visible and readable
• Provide final assets in a format suitable for docs site embedding (e.g. mp4 + thumbnail)

Acceptance Criteria

• Videos can be embedded into the docs under Getting Started
• Workflow shown matches current docs and does not require cloning the repo for install/inference