docs: document signed URL pattern lookup opt-out

[0xtlt]

🔗 Linked issue

Code PR: adonisjs/http-server#129
Discussion: https://github.com/orgs/adonisjs/discussions/5126

❓ Type of change

• 🐞 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)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This PR documents how to generate signed URLs from route patterns without performing a route lookup.

It adds an example for using signedUrlFor with disableRouteLookup: true, matching the behavior added in adonisjs/http-server#129.

Example:

signedUrlFor('/files/:key', { key: 'invoice.pdf' }, {
  disableRouteLookup: true,
  prefixUrl: 'https://example.com',
  qs: {
    download: true,
  },
})

When disableRouteLookup is enabled, the first argument is parsed as a route pattern and signed directly. Regular signedUrlFor calls still use route lookup and keep their route-name type safety.

📝 Checklist

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