Improve README with NYC taxi example and add examples directory

[unexge]

This PR improves the README documentation and adds an examples directory:

Changes

• README.md: Updated with a comprehensive, real-world example using NYC taxi trip data

  • Shows static typing with readColumn
  • Shows nullable column handling
  • Shows dynamic typing with readColumnDynamic
  • Includes expected output

• examples/nyc_taxi.zig: New runnable example that can be executed with zig build nyc-taxi

• build.zig: Added nyc-taxi build step for the example

• src/main.zig: Minor cleanup (removed debug allocator deinit assertion)