Document the GLB and SPZ splat formats

[mvaligursky]

Documents engine support for loading gaussian splats from glb files using the KHR_gaussian_splatting glTF extension (added in playcanvas/engine#9015), and from Niantic SPZ files via the external parser script (added in playcanvas/engine#9018).

⚠️ Do not merge until the engine release containing playcanvas/engine#9015 and playcanvas/engine#9018 ships — the pages document new public API (ContainerResource#gsplats, container-based splat loading) and the new spz-parser.mjs engine script.

Changes:

• New Splat File Formats → GLB page: how the extension stores splat data, differences from PLY (activated values, Y-up coordinate system, COLOR_0 fallback), creating files with SplatTransform, loading via container assets with a code example, and engine support notes (ellipse kernel only, sortingMethod/projection ignored, SPZ compression extension not supported).
• New Splat File Formats → SPZ page: format overview, creating files with SplatTransform, loading via the SpzParser script with ZSTD wasm module registration (draco-style) with a code example, and engine support notes (v4 only, data stays quantized on the GPU, up to 3 SH bands, Y-up coordinate system).
• Formats index updated to five formats: new GLB and SPZ sections, comparison table columns and workflow steps.
• Supported asset formats page: added glb and spz rows for splat data.
• SplatTransform format table: noted that .glb output is now loadable directly by the engine.
• Japanese translations for all of the above.