[examples] Fix CDN live preview demo

[oliviertassinari]

Problem

https://github.com/mui/material-ui/blob/master/examples/material-ui-via-cdn/README.md preview is broken.

Solution

See https://raw.githack.com/faq#diff-between-rawgithack-and-cdn for why this fixes the problem. Since it's only for demo purposes, we don't care about having our demo production ready.

Requests to CDN are routed through CloudFlare's content delivery network, and are cached for an year the first time they're loaded. This results in the best performance and reduces load on raw.githack.com and on underlying service, but it means that reloading won't fetch new changes

Context

It's another follow-up on #42172. The problem got more visible with v6.x release. I noticed this trying some of the examples that we have (since it's a critical part of the user journey, wanted to experience it as a new user).

Since it was quick to fix, I skipped the opening an issue step and went to PR (It will be better to only open issues in the future, but they tend to be not solved, e.g. #42423 so this should be more effective, until we figure a way to make it work).

[mui-bot]

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https://deploy-preview-43641--material-ui.netlify.app/

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