docs: update Nuxt version references and improve getting started guides

[nicokempe]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update (non-breaking change, improves clarity)

Description

This PR updates multiple documentation pages to ensure references and examples are up to date:

• Introduction page:
  • Replace outdated Nuxt 3 link (v3.nuxtjs.org) with canonical nuxt.com.
  • Adjust text to be version-agnostic (“Nuxt documentation”).
• Home page features:
  • Update feature title from “Nuxt 3 ready” to “Nuxt 3 and 4 ready”.
  • Update description to mention support for both Nuxt 3 and Nuxt 4.
• useSupabaseClient composable docs:
  • Update wording in useAsyncData example to remove the “Nuxt 3” qualifier.

These changes make the docs more version-agnostic and aligned with the canonical Nuxt domain.

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes (if not applicable, please state why)

[netlify]

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Name	Link
🔨 Latest commit	b54e0b4

[pkg-pr-new]

npm i https://pkg.pr.new/@nuxtjs/supabase@526

commit: b54e0b4

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[larbish]

🙏