Update README and website documentation for new API

[chtitux]

This commit updates all documentation to reflect the new unified filter-based API:

1. README.md Updates:

   • Added comprehensive Migration Guide section explaining the API changes
   • Updated all code examples to use new filter-based API
   • Fixed Complete Example section to use new methods
   • Cleaned up API Reference section to show current methods only
   • Added array filtering examples
   • Added documentation for new GTFS-RT time/delay fields
   • Removed references to deprecated getXXXByYYYY methods

2. Website Demo Updates:

   • Updated demo.ts to use new filter-based API
   • Fixed getStopById and getRouteById calls

Key improvements in documentation:

• Clear migration guide for users upgrading from older versions
• Examples showing both single value and array filtering
• Documentation of new GTFS-RT features (time and delay fields)
• Consistent examples throughout the README

All tests passing ✅