docs(plugins): document API 29 canvases and service menus

[blackbartblues]

Summary

Documents the plugin API 29 capabilities introduced by noctalia-dev/noctalia#4038:

• service.openContextMenu(request) for IPC-triggered menus at the cursor
• free-form panel nodes with position = "absolute", x, y, zIndex, and clipChildren
• target-local onDrop(payload, value, x, y) coordinates
• onPress on ui.input
• decorated = false for plugin-owned panel surfaces

Details

• adds API 29 to the central plugin API ledger
• supports separate service-context-menu and freeform-canvas badge keys for the same cumulative API level
• explains lifecycle, callback, compositor-independence, and dismissal behavior for service menus
• includes a complete sticky-note canvas example and migration-safe drag-and-drop guidance
• records the default decorated behavior and the exact visual frame removed by decorated = false

API 28 panel context menus are already documented by #226 and are not duplicated here.

Testing

• npm run build
• verified generated API ledger and API 29 badges in the built pages

Dependency

This documentation should merge with or after noctalia-dev/noctalia#4038.