docs(datetime-api): add Standard Time Zones reference diagram

What
- Added `Standard Time Zones.png` as part of datetime learning resources.
- Visual reference illustrating global standard time zones and offsets.

Why
- Helps understand how `ZoneId` and `ZonedDateTime` in Java map to real-world time zones.
- Useful for developers dealing with scheduling, logging, or globalized applications.

How to use
- Open `Standard Time Zones.png` to quickly identify zone offsets relative to UTC.
- Can be used alongside Java’s `ZoneId.getAvailableZoneIds()` to map IDs to actual offsets.

Real-life applications
- Handling international time zones in flight booking, financial trading, or calendar apps.
- Debugging time-related bugs in distributed systems across multiple regions.
- Teaching or documenting timezone concepts in Java projects.

Notes
- Prefer `ZoneId` (e.g., `ZoneId.of("Asia/Kolkata")`) over manual offset handling.
- This image complements examples from `DateTimeFormatDemo` and `CalendarDemo`.

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

Author: Someshdiwan