docs: import demos

[andretchen0]

This closes #242 .

• Since the demo source wasn't written to be viewed directly, there was often more code in them than absolutely necessary. I tried to take as much out as I could while maintaining the look of the demo – e.g., removing <TresCanvas v-bind="gl" /> and gl object and replacing it with <TresCanvas clear-color="#333">, or removing refs. I don't think that's noticeable for the most part, but let me know if I went too far.
• In a few places, I couldn't shorten the demos and maintain the visual effects or functionality, so I left the copy/pasted source code snippets. BackdropDemo.vue is one instance. (Note that it is possible to include a VSCode-style region, but I didn't want to complicate things. Only whole files are imported at the moment.)
• Ideally, code highlighting would be specified in the.vue demo files, not in the .md files. That's possible with the current system. But when the .vue files are parsed for rendering, they throw an error if used within a Vue component, e.g.:

<Box
   color="orange" // [!code hl]
   :size="1"
/>

Highlighting a component over multiple lines is a really common case for us. Since that breaks the demos, to keep things consistent, I left all highlighting in the .md files. That looks like this:

<<< @/.vitepress/theme/components/HtmlOccludeDemo.vue{2,6,19,28}

Above the lines 2, 6, 19, and 28 are highlighted.

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[JaimeTorrealba]

Thank you so much for this PR, solutions like this help us to scale more

I know is not specially fun but avoid a lot of typos problems

🙏🏼

[JaimeTorrealba]

And don't worry about the moment, we can use the '<<< @/.vitepress' Format.

This is already a great DX change for us. We could always go back to this later

[JaimeTorrealba]

I approved already, but before merge, let me know when you delete the .d.ts file please

[andretchen0]

Thank you so much for this PR, solutions like this help us to scale more

I know is not specially fun but avoid a lot of typos problems

I don't mind. It's better than trying to keep a copy of the demos in sync.

The next big missing piece for me is the props section of the docs. It'd really be great if those tables could be pulled from the components. I think most of the component have JSDocs for the props.

Ideas are welcome! 😀

[JaimeTorrealba]

Yes, as you can see we're a small numbers of maintainers here, a cientos was basically maintained just by me (Alvaro helps a lot, but he has other pkgs too, and Tino is full focus on the post-processing pkg)

So your addition is really appreciated, we could talk about how improve the pkg and the DX (not only do components)

Also, about another pkg for the ecosystem, I send to you a discussion in Discord about other future pkg

If you want, I'm available for discussing more on discord :)