Catalog intake workflow (issue form → PR → site)

[nikosarcevic]

Build an intake that turns a submitted form into a PR that appends _data/catalogs.yml.

• Issue form .github/ISSUE_TEMPLATE/catalog_submission.yml
• Action to validate & create PR (.github/workflows/catalog-intake.yml)
• Docs: how to submit

Acceptance: Filing the form creates a PR that, when merged, updates the Catalogs page.