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

What
- Introduced `Time Zones.png` to the project documentation.
- Provides a visual representation of global time zones across different regions.

Why
- Complements Java’s `ZoneId` and `ZonedDateTime` usage by giving a clear picture of real-world time zone distribution.
- Helps developers quickly relate Java’s time zone identifiers (e.g., "Asia/Kolkata", "America/New_York") to their geographical context.
- Supports better understanding for applications requiring cross-region scheduling, logging, and data consistency.

How to use
- Refer to `Time Zones.png` when working with Java’s `ZoneId.getAvailableZoneIds()`.
- Cross-check offsets and regions while building time-aware applications.
- Useful for documentation, teaching, or onboarding new developers.

Real-life applications
- Flight and travel booking systems (handling departures/arrivals in different zones).
- International financial applications (stock exchanges, forex).
- Globalized software (meeting schedulers, messaging apps, CRMs).

Notes
- This complements `Standard Time Zones.png` and `Core Date and Time Classes.png` diagrams already included.
- Encourages best practices: always use `ZonedDateTime` instead of manual offset calculations.

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

Author: Someshdiwan