Update Gaussian Splatting section and add AGENTS.md

[willeastcott]

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[Copilot]

Pull request overview

This pull request restructures the Gaussian Splatting documentation by flattening the directory structure and consolidating content. The engine-features subdirectory is removed, and its content is moved directly under the building directory. The standalone streaming-lod-editor.md file is removed, with its content incorporated into lod-streaming.md.

Changes:

• Flattened documentation structure by removing the engine-features subdirectory
• Consolidated streaming LOD documentation into a single file
• Expanded documentation for shadows, picking, performance, LOD streaming, custom shaders, and other engine features
• Added AGENTS.md file with guidelines for AI agents working on the documentation
• Updated all internal links to reflect the new file paths

Reviewed changes

Copilot reviewed 27 out of 32 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
AGENTS.md	Adds comprehensive guidelines for AI agents, including project structure, critical rules, and commands
sidebars.js	Updates navigation to flatten structure by removing engine-features category
docs/user-manual/gaussian-splatting/building/*.md	Creates/expands documentation files with detailed content (shadows, picking, performance, LOD streaming, custom shaders, draw order, index)
docs/user-manual/gaussian-splatting/building/your-first-app/*.md	Updates links to reference new flattened paths
docs/user-manual/gaussian-splatting/building/global-sorting.md	Updates internal links to new paths
docs/user-manual/gaussian-splatting/building/streaming-lod-editor.md	Removes file; content merged into lod-streaming.md
docs/user-manual/gaussian-splatting/building/engine-features/*.md	Removes old files; content moved to parent directory
i18n/ja/docusaurus-plugin-content-docs/current/user-manual/gaussian-splatting/building/*.md	Japanese translations for all new/updated English documentation

Comments suppressed due to low confidence (1)

docs/user-manual/gaussian-splatting/building/global-sorting.md:54

• There's an inconsistency in the domain used for live examples. This file uses playcanvas.vercel.app while other files in the same directory (e.g., shadows.md, picking.md, lod-streaming.md) use playcanvas.github.io. The domain should be consistent across all examples in this section.

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