Unified Splat Rendering documentation and restructure Gaussian Splatting section

[mvaligursky]

TO BE MERGED WHEN ENGINE 2.16 is out

• Adds comprehensive documentation for Unified Splat Rendering, a beta feature enabling global sorting and advanced gsplat features
• Restructures the Gaussian Splatting section with a clearer architecture overview
• Removes deprecated Full Shader Replacement documentation from custom-shaders page
• Adds note clarifying custom-shaders page is for non-unified rendering only