docs: Reorganize documentation by topics

[kyamagu]

Summary

Reorganize documentation by splitting the monolithic usage.rst (1,160 lines) into 5 focused topic-based files organized by skill level.

Changes

New Documentation Files

1. getting_started.rst - Introduction to the three main functions (match, sort, list) with basic examples
2. common_patterns.rst - Cookbook with practical examples for common font search tasks
3. advanced_topics.rst - Low-level API usage, patterns, charsets, and configuration management
4. properties_reference.rst - Complete font properties reference table
5. troubleshooting_reference.rst - Error handling and common issues

Modified Files

• index.rst - Updated toctree and cross-references to point to new files

Deleted Files

• usage.rst - Original monolithic file removed after content migration

Benefits

• ✅ Better navigation: Users can jump directly to their skill level
• ✅ Easier maintenance: Changes isolated to specific topic areas
• ✅ Improved discoverability: Clear file names indicate content
• ✅ Better learning flow: Sequential progression from beginner to advanced
• ✅ Cleaner reference: Properties and troubleshooting in dedicated files

Verification

• All content from original usage.rst has been migrated
• Cross-references updated and verified
• No broken links
• Documentation structure follows skill-level organization (beginner → advanced)

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