Promotes the VR LOD Gaussian Splatting example from hidden/standalone to a visible example within the Gaussian Splatting section, with a brief description and updated thumbnails.

Changes:

• Moved vr-lod example + controls from gaussian-splatting-xr/ into gaussian-splatting/, removing the now-empty folder
• Added a short description: "Demonstrates Gaussian Splat rendering in VR (WebXR)"
• Removed the HIDDEN flag so the example appears in the browser
• Swapped scene order so the ice cave loads first (church is now scene 1)
• Regenerated thumbnails from the ice cave screenshot
• Moved the in-iframe "Enter VR" button to center-bottom so it no longer sits under the description overlay

Examples:

• Gaussian Splatting → VR LOD (newly visible)