Examples: Add material.alphaHash example

[donmccurdy]

Adds example for material.alphaHash, introduced in #24271.

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Unfortunately I'm having trouble getting the chromium/puppeteer screenshots to run on my M1 MacBook Pro. Output:

Using Chromium r1108766, stable channel on mac_arm
Downloading...
Downloaded.
Error: webgl_materials_alphahash: THREE.WebGLRenderer: Error creating WebGL context., another attempt...

Other examples are failing too, so perhaps an issue with my computer or the current Chromium release. Could someone else try generating the screenshot?

[Mugen87]

Could someone else try generating the screenshot?

Done! I've used a Windows laptop to download your branch and generate the screenshot.