feat(datetime): add CalendarDemo using GregorianCalendar

What
- Implemented CalendarDemo class to demonstrate usage of java.util.GregorianCalendar.
- Showcased methods for:
  • Checking leap year with isLeapYear().
  • Getting current date, month, day of week, and day of month.
  • Printing full calendar object details.

Why
- Demonstrates how the legacy Calendar API works in real-world scenarios.
- Useful for understanding date and time handling in pre–Java 8 applications.
- Highlights GregorianCalendar as the commonly used calendar system (worldwide standard).

Notes
- Calendar.DATE → current day of the month.
- Calendar.MONTH → zero-based index (January = 0).
- Calendar.DAY_OF_WEEK → values 1–7 (Sunday = 1).
- Calendar.DAY_OF_MONTH → same as DATE but explicitly named.
- isLeapYear(year) → checks if the given year is a leap year.

Next Steps
- Add examples of manipulating dates (add/subtract days, months).
- Compare with java.time.LocalDate for modern, immutable, and thread-safe usage.

Signed-off-by: https://github.com/Someshdiwan <[email protected]>

Author: Someshdiwan

Section26DateandTimeAPI/src/CalendarDemo.java renamed to Section26DateandTimeAPI/Calendar/src/CalendarDemo.java

@@ -4,8 +4,8 @@
 public class CalendarDemo {
     public static void main(String[] args) {
         //Gregorian Calendar we follow.
-
         GregorianCalendar gc = new GregorianCalendar();
+
         System.out.println(gc.isLeapYear(2020));
 
         System.out.println("Today Date: "+gc.get(Calendar.DATE));
@@ -16,4 +16,4 @@ public static void main(String[] args) {
 
         System.out.println(gc);
     }
-}
+}