docs: clean demos

[damienmontastier]

This PR cleans up the usage demos in the documentation (those from version >V2.0) to improve readability. Although the demos have been streamlined, it was still essential to provide users with the complete source code. To achieve this, an expandable menu has been added below each demo, allowing users to view the demo's code.

I've also modified the demos in the documentation introduction. The tags were missing.

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Are demos still too massive, or is that okay?

Because the imports can be removed, I just think it's important to keep <TresPerspectiveCamera /> to understand the level at which to activate <EffectComposerPmndrs />.

I haven't modified @Tinoooo demos (before version 2.0), I may do so later when we've agreed on a “basic model” of demo usage style.

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The branch is deliberately based on the fix/bad-use-multisampling branch. I also based the PR directly on the bad-use-multisampling branch instead of main. I don't know if that's a good practice or not, you tell me!