Docs: Move `demo` into `docs`

[Tahul]

I used to maintain this website showcasing capabilities: https://vueuse-motion-demo.netlify.app

At the time it was introduced, none of @vueuse/motion was built with Nuxt nor @nuxt/content.

Now that we have MDC and syntax coloring from @nuxt/content, I would love to have some great examples for the usage "from the code" with a nice sandbox showcasing it close.

Much like we do for presets but with a bit more of code!

WDYT @cpreston321 @alex-eliot?

[peoray]

Could you provide more context? I'd like to help out here for my first time
cc @BobbieGoede @Tahul