docs(cloudflare): add environment variables instructions

[Hebilicious]

🔗 Linked issue

Resolves #272
Extracted from #1318

❓ Type of change

• 📖 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

Document the usage of environment variables and useRuntimeConfig in cloudflare presets.
useRuntimeConfig(event) only works with unenv.

📝 Checklist

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

[codecov]

Codecov Report

Merging #1547 (96b9f89) into main (a949cb9) will not change coverage.
Report is 41 commits behind head on main.
The diff coverage is n/a.

❗ Current head 96b9f89 differs from pull request most recent head d1d8ef4. Consider uploading reports for the commit d1d8ef4 to get more accurate results

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[Hebilicious]

@pi0 A new section is in #1550, this PR only contains cloudflare specific instructions

[Hebilicious]

@pi0 When you get the time to look at this, could you also clarify why/how we can use useRuntimeConfig() without event in defineLazyEventHandlers ? This pattern isn't documented anywhere.

[pi0]

@Hebilicious

export default lazyEventHandler(() => {
  // Access here
  return eventHandler(() => {
    
  })
})

We can mention it in both routing and config sections.