doc: Update link to cloud-init user data format

[victorlin]

The old link no longer works, and the relevant content has been moved to a more specific page.

Error flagged by scheduled CI run:

Broken links found:
{
  "filename": "aws-batch.md",
  "lineno": 535,
  "status": "broken",
  "code": 0,
  "uri": "https://cloudinit.readthedocs.io/en/latest/topics/format.html",
  "info": "404 Client Error: Not Found for url: https://cloudinit.readthedocs.io/en/latest/explanation/format.html"
}

Related to canonical/cloud-init#6730

Checklist

• linkcheck passes
• Update changelog

[victorlin]

Merging – link check passes and failing checks seem unrelated.