[docs] Migrate content to the new location

[siriwatknp]

Ahrefs result: https://app.ahrefs.com/site-audit/3276941/5/overview

Current URL: https://deploy-preview-30757--material-ui.netlify.app/components/buttons/
New URL: https://deploy-preview-30757--material-ui.netlify.app/material/react-button/

Once this PR is merged, there might be some conflict on other PRs if they touch .md file. Those PRs have to revert the change and update the markdown in /docs/data/*.md instead.

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• I have followed (at least) the PR section of the contributing guide.

[mui-bot]

No bundle size changes

Generated by 🚫 dangerJS against 021a22c

[oliviertassinari]

A side note, I'm not a fan of this change:

-{{"demo": "pages/components/alert/TransitionAlerts.js"}}
+{{"demo": "TransitionAlerts.js"}}

Previously, you could grep the demo and search it in your IDE, zero conflicts. Now, there is a higher chance that the name will conflict with another folder.

[siriwatknp]

A side note, I'm not a fan of this change:

-{{"demo": "pages/components/alert/TransitionAlerts.js"}}
+{{"demo": "TransitionAlerts.js"}}

Previously, you could grep the demo and search it in your IDE, zero conflicts. Now, there is a higher chance that the name will conflict with another folder.

Let me state why I did this change:

1. there is a TODO in loader.js

    // TODO: const moduleID = demoName;
    // The import paths currently use a completely different format.
    // They should just use relative imports.
   

2. Using relative imports means no couple with path (the whole folder can be moved around with no change, I know that this is a very rare case but I like having things independent from each other.

Can you explain more about what did you usually search for? Maybe we can do something about it.