docs: add comprehensive documentation files

Added documentation for API, architecture, configuration, database,
development guide, testing, and navigation. Includes helper scripts,
diagrams, and guides for NFO files and migration.

Docs/diagrams/README.md

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+# Architecture Diagrams
+
+This directory contains architecture diagram source files for the Aniworld documentation.
+
+## Diagrams
+
+### System Architecture (Mermaid)
+
+See [system-architecture.mmd](system-architecture.mmd) for the system overview diagram.
+
+### Rendering
+
+Diagrams can be rendered using:
+
+-   Mermaid Live Editor: https://mermaid.live/
+-   VS Code Mermaid extension
+-   GitHub/GitLab native Mermaid support
+
+## Formats
+
+-   `.mmd` - Mermaid diagram source files
+-   `.svg` - Exported vector graphics (add when needed)
+-   `.png` - Exported raster graphics (add when needed)