docs: translate docs/AUTHORING.md key sections to Spanish

[ulises-jeremias]

Problem

Spanish-speaking contributors hit a wall reading AUTHORING.md (file conventions, EJS-like overlays, template/, partial pyproject).

Solution

Add docs/AUTHORING.es.md (or a Spanish section) covering: folder layout, templates.json fields, extension template/ overlay, docs append files, and local file:// testing. Link it from README/CONTRIBUTING.

Acceptance criteria

• Spanish doc exists and is linked
• English remains canonical
• Mentions CI L0–L3 briefly