feat(datetime-api): add DateTimeFormatDemo showcasing custom date-time formatting

What
- Introduced DateTimeFormatDemo under DateandTimeAPI package.
- Demonstrates use of DateTimeFormatter with LocalDateTime and ZonedDateTime.
- Added multiple formatting patterns:
  - "dd/MM/yyyy" → formats date only (day/month/year).
  - "dd/MM/yyyy hh:mm:ss" → adds time in 12-hour format.
  - "dd/MM/yyyy hh:mm:ss z" → includes time zone abbreviation using ZonedDateTime.

Why
- Provides clear examples of formatting temporal objects using patterns.
- Shows how different symbols (y, M, d, h, H, m, s, S, z, Z) control output formatting.
- Reinforces distinction between LocalDateTime (no timezone) and ZonedDateTime (timezone aware).

How to use
- Run DateTimeFormatDemo.
- Output will display:
  1. Current system date in dd/MM/yyyy.
  2. Current system date + time in dd/MM/yyyy hh:mm:ss.
  3. Current zoned date-time including timezone (e.g., IST, PST).

Real-life applications
- Formatting timestamps in logging frameworks.
- Displaying human-readable dates in UIs, reports, or invoices.
- Parsing/printing ISO or custom date formats in APIs.
- Handling time zones for global applications.

Notes
- `hh` prints 12-hour format; use `HH` for 24-hour.
- `z` gives short timezone name (e.g., IST), `Z` gives numeric offset (e.g., +0530).
- Thread-safe: DateTimeFormatter is immutable and safe for concurrent use.

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

Author: Someshdiwan

Section26DateandTimeAPI/src/DateandTimeAPI/DateTimeFormatDemo.java

@@ -19,6 +19,7 @@ public static void main(String[] args) {
         //S = milliseconds
         //z = time zone
         //Z = offset
+
         LocalDateTime dt = LocalDateTime.now();
         DateTimeFormatter df = DateTimeFormatter.ofPattern("dd/MM/yyyy");
         System.out.println(df.format(dt));