docs: link demos to playground (beta)

[OrbisK]

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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
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⚠️ Slowing down new functions

Warning: Slowing down new functions

As the VueUse audience continues to grow, we have been inundated with an overwhelming number of feature requests and pull requests. As a result, maintaining the project has become increasingly challenging and has stretched our capacity to its limits. As such, in the near future, we may need to slow down our acceptance of new features and prioritize the stability and quality of existing functions. Please note that new features for VueUse may not be accepted at this time. If you have any new ideas, we suggest that you first incorporate them into your own codebase, iterate on them to suit your needs, and assess their generalizability. If you strongly believe that your ideas are beneficial to the community, you may submit a pull request along with your use cases, and we would be happy to review and discuss them. Thank you for your understanding.

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Description

This PR adds an extra link to https://playground.vueuse.org with the current demo as the welcome SFC. I consider it beta because we sometimes import "random" stuff that does not currently work in Playground.

Additional context

[OrbisK]

/cc @antfu

FYI: pnpm does autoformats like this when using e.g. pnpm patch. Maybe we should leave it like this to prevent future noise/merge conflicts. (just talking about the blank lines btw 😅)

[antfu]

Let's go, thanks for working on it!