Add CustomMaterial and PBRCustomMaterial docs

[grantf04]

Summary

• add a Custom Materials page under the materials library for CustomMaterial and PBRCustomMaterial
• document setup, shader hook points, uniforms, and common usage patterns
• add two verified Playground demos and generated preview images
• add the page to the materials library index and navigation structure

Bounty reference: BitReelCo/BJS-PR-Bounty-Pool#5

Verification

• created fresh Playground demos: #QF2QWZ#0 and #W0OF0A#0
• opened both demos in the Babylon Playground with Playwright and confirmed they render without console errors
• node -e "JSON.parse(require('fs').readFileSync('configuration/structure.json','utf8'))"
• serialized content/toolsAndResources/assetLibraries/materialsLibrary/customMat.md through the repo MDX pipeline
• npm run build

Notes

• npm test currently fails on existing snapshot drift in the baseline suite: KaTeX math output and content/whats-new.md snapshot content. Those failures are unrelated to this docs page.

[RaananW]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[bjsplat]

snapshot for this PR available at:

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/DOCS/refs/pull/1574/merge

[Copilot]

Pull request overview

Adds documentation for Babylon.js CustomMaterial and PBRCustomMaterial within the Materials Library, and wires the new page into the Materials Library index and site navigation.

Changes:

• Added a new Materials Library page covering setup, shader hook points, and uniform usage for CustomMaterial and PBRCustomMaterial, including two Playground examples.
• Updated the Materials Library index page to include a link to the new Custom Materials documentation.
• Updated navigation structure to include the new page under the Materials Library section.

Reviewed changes

Copilot reviewed 3 out of 5 changed files in this pull request and generated no comments.

File	Description
content/toolsAndResources/assetLibraries/materialsLibrary/customMat.md	New docs page describing CustomMaterial/PBRCustomMaterial usage, hook points, uniforms, and Playground examples.
content/toolsAndResources/assetLibraries/materialsLibrary.md	Adds a list of Materials Library pages, including the new Custom Materials link.
configuration/structure.json	Adds customMat entry so the new page appears in the navigation tree.

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[RaananW]

Hey @grantf04 , thank you so much for the PR. I have no problem with the content, I just want to say that the bounties were external to the Babylon team, so i can't guaranty anything in that regard. I also noticed that the bountry repository was archived.

I will ask someone from the team to fully review thins, but it does LGTM!