Document gsplat fragment shader customization and relighting

[mvaligursky]

!!!!!!!! MERGE WHEN engine v2.20.0 is out !!!!!!!!!

Documents the gsplat fragment shader customization (gsplatModifyPS) and the relighting technique added in playcanvas/engine#8872.

Changes:

• Split the Custom Shaders page into a section: a main concepts page (the two hooks, choosing a stage, shared apply/remove patterns) with Vertex Stage and Fragment Stage child pages. The main page keeps the /custom-shaders URL; children live at /custom-shaders/vertex and /custom-shaders/fragment.
• New Fragment Stage page documenting the gsplatModifyPS chunk: modifySplatColor signature (GLSL/WGSL), call point and parameters, available inputs, and a screen-space tint example.
• New Relighting page: lighting splat scenes with standard lights via a proxy mesh and the GsplatRelighting script — concept, setup code, attribute reference and tuning notes, with the live example embed.
• Work Buffer Rendering page links to the new fragment customization; sidebar gains the Custom Shaders category and the Relighting entry.

Both live example embeds reference the new gaussian-splatting/relighting engine example, which resolves once the engine release is deployed.