docs: expand Leash site with current product surface

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Summary

• keep the Leash page concise and product-first rather than mirroring the README
• add the Geek on a Leash demo as a prominent path
• add agent workflows, bounded navigation, CPU/CUDA processing, ROS 2, hardware, and the current safety boundary
• preserve the existing site style and quick-start flow

This is a selective landing-page update, not a word-for-word README copy.