feat(datetime-api): add CalendarDemo showcasing GregorianCalendar usage

What
- Added CalendarDemo under DateandTimeAPI package.
- Demonstrates usage of java.util.GregorianCalendar.
- Highlights key operations:
  - Checking leap years with isLeapYear().
  - Retrieving today’s date, month, day of week, and day of month.
  - Printing the full GregorianCalendar object.

Why
- Provides a practical example of working with the legacy Calendar API.
- Helps understand how GregorianCalendar handles common date/time queries.
- Builds a foundation for comparing legacy APIs with modern java.time (introduced in Java 8).

How to use
- Run CalendarDemo.
- Output will display:
  1. Leap year check (true for 2020).
  2. Current day of month (DATE).
  3. Current month (0-based: January=0, December=11).
  4. Day of the week (1=Sunday, 7=Saturday).
  5. Current day of month again (DAY_OF_MONTH).
  6. Complete calendar object with full internal state.

Real-life applications
- Scheduling events and checking leap years in scheduling software.
- Payroll systems calculating month-end dates.
- Banking/finance applications handling due dates and weekends.
- Ticket booking systems validating weekdays and weekends.

Notes
- `Calendar.MONTH` is zero-based (January=0).
- `Calendar.DAY_OF_WEEK` returns 1–7 with Sunday as 1.
- GregorianCalendar is mutable and not thread-safe.
- For modern applications, prefer `java.time` classes (LocalDate, LocalDateTime, ZonedDateTime).

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

Author: Someshdiwan

Section26DateandTimeAPI/Calendar/src/CalendarDemo.java

@@ -7,8 +7,8 @@ public static void main(String[] args) {
         GregorianCalendar gc = new GregorianCalendar();
 
         System.out.println(gc.isLeapYear(2020));
-
         System.out.println("Today Date: "+gc.get(Calendar.DATE));
+
         System.out.println(gc.get(Calendar.MONTH));
 
         System.out.println(gc.get(Calendar.DAY_OF_WEEK));