Add pure barrel tree-shaking docs

[RaananW]

Summary

• Add a new ES6/NPM Support child section for tree-shaking with pure imports
• Document the pure barrel workflow, explicit side-effect registration, troubleshooting, and contributor patterns
• Include complete pure-import examples for an animated glTF scene, Physics V2 with Havok, and WebXR

Validation

• Parsed configuration/structure.json
• Ran git diff --check
• Checked VS Code diagnostics for the new tree-shaking docs pages

Notes

• CheckMissingImports is documented with its direct diagnostic import path because it is not exported from @babylonjs/core/pure.
• Earlier full npm run build completed successfully during prep, with existing static generation warnings/timeouts that did not fail the build.

[bjsplat]

snapshot for this PR available at:

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/DOCS/refs/pull/1572/merge

[bjsplat]

snapshot for this PR available at:

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/DOCS/refs/pull/1572/merge