docs: guide accessible progressive labels for generated SVG diagrams

[ArianMoeini]

Summary

Add a working progressive-label pattern to both SVG diagram guides used by OpenGenerativeUI. Generated mechanical diagrams can keep short part names visible while revealing longer descriptions on pointer hover or keyboard focus, with a tap-friendly details list for touch users.

This follows the approach I proposed on #41. It changes generation guidance rather than the sandbox renderer; I welcome maintainer direction if the issue calls for a different layer.

Changes

• Add an example diagram with readable labels and reserved space for detail text.
• Provide keyboard and screen-reader names, a touch fallback, dark palette, and horizontal scrolling on narrow screens.
• Keep the agent and MCP guidance in sync.

Verification

• Checked pointer hover, keyboard focus, touch disclosure, narrow-screen scrolling, and dark palette in a browser.
• Confirmed both guide copies match and git diff --check passes.

Refs #41