docs: use random in strands dithering example

[puneetdixit200]

Summary

• updates the buildStrokeShader dithering example to use random() instead of noise() for per-pixel random masking
• keeps continuous noise() examples unchanged where coherence over position/time is intended

Fixes #8776.

Testing

• npm ci
• npm run docs
• git diff --check
• npx eslint src/webgl/material.js (exits 0; existing warnings remain in the file)

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Thank You!

[perminder-17]

Hi, thanks for the work and for taking the time to open this PR.

I’ll need to close this PR because our contributing guidelines require contributors to get assigned to an issue before starting work on it, as mentioned here:
https://github.com/processing/p5.js/blob/dev-2.0/CONTRIBUTING.md#get-assigned-before-working-on-an-issue

Also, another contributor had already expressed interest in working on this issue earlier, so I’ll be assigning it to them first.