feat: enhance documentation

[allisson]

Add extensive documentation improvements across the entire codebase:

• Add comprehensive package-level documentation to query.go explaining library purpose, basic usage, filter syntax, and supported database flavors
• Enhance all 7 public function docs with detailed parameter descriptions, return value explanations, and practical code examples
• Document all 4 private helper functions (parseIn, parseSelectFilter, parseUpdateFilter, parseDeleteFilter) explaining their purpose and behavior
• Improve type documentation in options.go with detailed field descriptions for all 4 option types (FindOptions, FindAllOptions, UpdateOptions, DeleteOptions)
• Enhance all 16 method documentation with clearer descriptions and usage guidance
• Completely rewrite README.md with 30+ code examples covering:
  • Basic usage for SELECT, INSERT, UPDATE, DELETE operations
  • Advanced filtering with complete operator reference table
  • Pagination and ordering examples
  • Row locking (FOR UPDATE) with various modes
  • Struct-based query building with tag examples
  • Database flavor handling for MySQL, PostgreSQL, SQLite
  • Four complete real-world examples (user management, e-commerce order processing, data cleanup, product search)

The documentation now provides developers with clear, copy-paste ready examples for every feature, making the library significantly more accessible and easier to use.