[Open Bounty] ( $100 ) - Create documentation and demos for Custom Materials including Custom PBR

[br-matt]

[PirateJC]

Write documentation for the the Babylon Documentation pages, detailing and describing the custom material.

It should be located in the materials library section of the documentation:
https://doc.babylonjs.com/toolsAndResources/assetLibraries/materialsLibrary

It should take the core information here: https://github.com/BabylonJS/Babylon.js/blob/master/packages/dev/materials/src/custom/readme.md#L0-L1

And walk users through the features.

It also should include playground demos to highlight the features and clearly show how to use them.

[grantf04]

Submitted PR for this bounty: BabylonJS/Documentation#1574

Scope covered:

• New Custom Materials page in the materials library section
• CustomMaterial and PBRCustomMaterial setup and usage
• Uniforms and shader hook-point documentation
• Two fresh Playground demos for CustomMaterial and PBRCustomMaterial
• Navigation/index wiring and generated preview images

Verification included in the PR: both Playground demos render without console errors, the new page serializes through the docs MDX pipeline, and npm run build passes.

Please treat this as my submission/claim for the $100 bounty.