Add basic docs building system. Most of the requirements and fixes directly landing to main branch.

Checklist:

• Docs initial structure
• Docs initial design
• Local development
• Production build
• Deploy (https://framework-git-docs-nuxt-js.vercel.app/)

Browse: https://github.com/nuxt/framework/tree/docs/docs

Not in this PR:

• Untyped loader
• Finished design
• docs contents
• HMR experience
• Remote fetching (instead of using bundled assets to avoid redeploy on content update)

Note: Please do not use current implementation externally. It is pretty much a playground to have something for docs and finally, they need to be encapsulated to a maintainable package.