docs: Add ARCHITECTURE.md and update AGENTS.md for AI context

[erseco]

This PR introduces architectural documentation to better guide both AI agents and human developers working on the Documentate plugin.

Changes Included:

1. Added ARCHITECTURE.md: A high-level document that covers:
   • The primary components: documentate_document CPT and documentate_doc_type taxonomy.
   • The Document Generation flow using OpenTBS and .odt templates.
   • The Document Conversion flow (Collabora Online vs. WASM ZetaJS).
   • Access Control and Scope logic (filtering by user scope categories).
   • Potential areas of improvement and notes on the REST API protection granularity.
2. Updated AGENTS.md: Added a directive instructing AI agents to review ARCHITECTURE.md before attempting any major modifications to ensure architectural consistency.

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PR created automatically by Jules for task 401451429682461264 started by @erseco

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