docs: correct useApp camera lookup

[abstrakt8]

Summary

• replace app.scene.camera in useApp docs with a correct camera component lookup
• Fixes the confusion caused in app.scene.camera is undefined react#311
• https://api.playcanvas.com/engine/classes/Scene.html

[Copilot]

Pull request overview

This PR corrects the documentation for the useApp hook by replacing an incorrect camera lookup pattern with the proper method for accessing camera components in PlayCanvas.

Changes:

• Fixed incorrect app.scene.camera reference to use the correct app.root.findComponents('camera') pattern
• Added explanatory comment clarifying that cameras are components on entities
• Updated description to include additional accessible properties (root entity, systems, assets, events)

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