Fix broken cloud research link

[PerceiveWorld]

Summary

• Replace the broken GGI Cloud Expertisecentrum research URL with the current VNG article that references the same 'Cloudgebruik en -ondersteuning gemeenten' research context.

Verification

• Confirmed the old URL returns HTTP 404.
• Confirmed the replacement VNG URL returns HTTP 200.
• Checked the compare diff only changes the cloud guideline markdown link.

Fixes #276