docs: move docs

[yuyinws]

🔗 Linked issue

https://github.com/unplugin/docs/issues/7

❓ Type of change

• [x] 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• [ ] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [ ] 👌 Enhancement (improving an existing functionality like performance)
• [ ] ✨ New feature (a non-breaking change that adds functionality)
• [ ] 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This PR includes:

1. Move unplugin/docs to this repo.
2. Add a new GitHub workflows file. Ensure that Vercel automatic build is triggered every time the main branch is updated.

📝 Checklist

• [x] I have linked an issue or discussion.
• [x] I have updated the documentation accordingly.

[yuyinws]

@sxzz Hi, Once this PR merge. I will dm you in discord about the GitHub action secrets like VERCEL_TOKEN.

[sxzz]

@antfu Could you please grant me the admin access of repo.

[sxzz]

I plan to move it to Netlify by using Anthony's or Unjs org's account, since we cannot manage the website in an organization if deployed in Vercel.

[yuyinws]

After the new site is available, I will let the old domain redirect to the new one.

[sxzz]

@yuyinws New docs have landed on https://unplugin.unjs.io/

We should also archive old repo

[yuyinws]

@sxzz Old repo's README has been update. But I have no access to archive it.

https://unplugin.vercel.app now redirect to the new one.

[sxzz]

Now archived. Thanks for your amazing work!