docs: restructure Cameras section into multi-page, workflow-agnostic docs

[willeastcott]

Summary

Restructures the Cameras section of the user manual from a single thin, Editor-focused page into a full multi-page section, following the workflow-agnostic pattern established by the shadows rewrite (#1037).

New section structure

Page	Content
index.md (rewritten)	Section overview: what cameras are, creating one (4-way workflow tabs), clearing the render target, section tour
projection.md (new)	Perspective vs orthographic, FOV/orthoHeight, clip planes & depth precision, aspect ratio, frustum culling
tone-mapping.md (new)	Per-camera tone mapping curves, gamma correction, physically based exposure (aperture/shutter/sensitivity)
multiple-cameras.md (new)	Priority ordering, viewports (split-screen), layers, camera stacking, render targets, performance notes
camera-controls.md (new)	The engine's camera-controls.mjs script (orbit/fly/pan) across all four workflows, key attributes
screen-and-world.md (new)	screenToWorld / worldToScreen patterns: mouse rays, object placement, HTML overlays

The existing scene-picker.md and depth-layer.md pages are unchanged and remain in the section.

Notes

• All pages use the <Tabs groupId="workflow"> pattern (Engine / Editor / React / Web Components) where implementation differs.
• Engine facts (property defaults, constants, script attributes) verified against current engine source.
• Live <EngineExample> embeds used instead of new screenshots (camera/multi, camera/orbit, camera/fly, graphics/render-to-texture, graphics/light-physical-units).
• Japanese translations mirrored for all six pages.
• Sidebar updated with the five new pages.
• Dropped the standalone camera-icon.png usage (viewport entity icons are now documented on the Viewport page); camera-create.png moved inside the Editor tab.

🤖 Generated with Claude Code