docs(datetime): add Core Date and Time Classes diagram

What
- Added `Core Date and Time Classes.png` illustrating the core classes in the Java Date and Time API (java.time).
- Shows the hierarchy and relationships among:
  - LocalDate, LocalTime, LocalDateTime (date and/or time without timezone).
  - ZonedDateTime, OffsetDateTime, OffsetTime (timezone/offset aware).
  - Instant (machine timestamp from epoch).
  - Period and Duration (human-readable vs machine-based differences).
  - Clock and ZoneId (time source and zone abstraction).

Why
- Provides a quick visual reference for understanding how the java.time API is structured.
- Helps differentiate between:
  - Human-centric classes (LocalDate, Period).
  - Machine-centric classes (Instant, Duration).
  - Zone/offset-aware classes (ZonedDateTime, OffsetDateTime).
- Useful for learning, teaching, and onboarding developers to modern Java date/time handling.

How to use
- Open `Core Date and Time Classes.png` to explore the class hierarchy.
- Refer to it while coding with `java.time` API to choose the right class for the use case.

Real-life applications
- Choosing the correct class when:
  - Storing only dates (e.g., birthdays → LocalDate).
  - Working with precise machine timestamps (e.g., logging → Instant).
  - Handling global scheduling (e.g., flights → ZonedDateTime).
  - Calculating time spans (e.g., subscriptions → Period, Duration).

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

Author: Someshdiwan