Docs: Update README with all supported calendars

Docs: Update README with all supported calendars The README.md has been updated to accurately document all calendar systems (Algorithms) supported by the library. Previously, the README primarily focused on the Hijri calendar and implied that only Hijri variants were planned. This update clarifies that Gregorian and Native (Unix timestamp-based) calendars are also fully supported. Changes include: - Revised introduction to mention all supported calendar types. - Added a new "Supported Calendars" section. - Provided detailed usage examples for: - `GregorianAlgorithm` - `HijriFatimidAstronomical` (existing example refined and relocated) - `NativeAlgorithm` - Each example shows instantiation, conversion from Julian Day, and conversion to Julian Day.