Mobile optimization: document VFX emitter and light source scene limits

The Godot Explorer scene stats overlay now surfaces 4 new per-scene rows (decentraland/godot-explorer#2674):

- **VFX emitters** — active particle emitters (soft 8 / hard 10)
- **Live particles** — sum of authored particle amounts (soft 25,000 / hard 50,000)
- **Light sources** — authored SDK LightSource components, active only (soft 8 / hard 16)
- **Shadow lights** — authored lights with shadows enabled (soft 2 / hard 4)

The mobile optimization page should document these budgets so creators understand why scenes that read green on legacy rows (triangles/entities/textures) can still drive session overheat on mobile.

Context worth including:
- Light rows show **authored** counts, not the runtime per-profile budget (Very Low 0 / Low 2 / Medium 4 / High 8 active lights — see decentraland/godot-explorer#2633). A scene can author 20 lights; the engine will only run 8 on High, but authoring beyond budget is wasted scene cost.
- Audit baseline: Genesis Plaza authors 59 lights / 48 shadow lights; rituals.dcl.eth 20 lights; doriangray.dcl.eth 16 lights.
- Runtime particle budgets already enforced: 5,000 per emitter, 50,000 per scene.

Refs: decentraland/godot-explorer#2674, decentraland/godot-explorer#2368, decentraland/godot-explorer#1538

Start with the mobile optimization page and the Godot Explorer scene stats overlay context in the issue. Document the four scene rows and their soft and hard limits, explain authored versus runtime light budgets, and include the stated particle limits and audit examples; done means creators can understand mobile overheat risks beyond legacy rows.

Written by the indexing model from the issue text.