Specify `templates.json` schema parity with cna-templates

[ulises-jeremias]

Summary

Document schema for templates and extensions registry.

CNA mapping

CNA: cna-templates templates.json as source of truth.

Acceptance criteria

• Schema doc under docs/ or docs/adr/
• Fields for categories, URLs, addons compatibility noted
• Differences from Node stack called out

Blocked by

none

Blocks

none

Reference

Parity matrix (workspace): knowledge/create-python-app-parity-matrix.md
Reference repo: https://github.com/Create-Node-App/create-node-app

Parent epic

#6