Meetup: Add hints.md to guide students towards TemporalAdjusters

The Java "Meetup" exercise requires calculating specific dates based on rules like "first Monday", "third Tuesday", "teenth Wednesday", or "last Friday" of a given month and year. While solvable by manually iterating through days, the idiomatic and most efficient solution leverages the java.time API, specifically the java.time.temporal.TemporalAdjusters utility class.

Many students, especially those less familiar with Java 8+ date/time features, may not be aware of TemporalAdjusters. This can lead to frustration or reliance on more verbose, less efficient manual implementations, potentially missing a key learning opportunity about the standard library.

## Proposed Solution:

Add a hints.md file to the exercises/practice/meetup/ directory to gently guide students towards the relevant java.time features without giving away the implementation details.