Add hints document for Meetup exercise

exercises/practice/meetup/.docs/hints.md

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+# Hints
+
+## The `java.time` Advantage
+
+Java's modern Date and Time API (`java.time` package, since Java 8) is particularly well-suited for this kind of date calculation exercise.
+
+## Managing Month and Year Context
+
+Your constructor receives the `month` and `year`. While you can use these directly, the `java.time` package offers a convenient class for handling this context:
+
+- [`YearMonth`](https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/time/YearMonth.html): Represents a specific month in a specific year. It's useful because it inherently knows the number of days in that particular month, automatically handling leap years. 
+
+## Calculating Specific Dates: `TemporalAdjusters`
+
+The core of this exercise involves finding specific dates based on rules (FIRST, SECOND, TEENTH, LAST, etc.). Instead of manually iterating through days and counting, the `java.time` API provides a powerful utility class designed for exactly these kinds of common date adjustments:
+
+- [`java.time.temporal.TemporalAdjusters`](https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/time/temporal/TemporalAdjusters.html) (note the plural 's').
+
+This class is key to solving the exercise idiomatically. Explore its `static` methods. You'll find pre-built adjusters for finding:
+- The first or last occurrence of a specific `DayOfWeek` within a month (e.g., `firstInMonth()`, `lastInMonth()`).
+- The Nth occurrence of a specific `DayOfWeek` within a month (e.g., `dayOfWeekInMonth()`).
+- The next occurrence of a specific `DayOfWeek` from a given date (e.g., `nextOrSame()`, which is helpful for the `TEENTH` case).
+
+## Applying Adjustments
+
+- A `TemporalAdjuster` (obtained from a static method in `TemporalAdjusters`) is typically used with the `.with(TemporalAdjuster)` method on a `LocalDate` instance. This method returns a *new* `LocalDate` reflecting the adjustment.