- Add Django management command 'export_json' for exporting all dokumente data
- Implement structured JSON format with proper section types from database
- Include all document fields: gueltigkeit, signatur_cso, anhaenge, changelog
- Support Kurztext, Geltungsbereich, Einleitung with Langtext-style structure
- Use actual abschnitttyp values instead of hardcoded 'text'
- Handle Referenz model fields correctly (name_nummer, name_text)
- Support --output parameter for file export or stdout by default
