Add documentation for the Meetup concept exercise

Introduce `introduction.md.tpl` and `hints.md` for the Meetup exercise to guide learners. The hints provide detailed steps on working with `LocalDate` and `TemporalAdjusters` for date manipulations.

Author: Jegors Cemisovs

exercises/concept/meetup/.docs/hints.md

@@ -0,0 +1,43 @@
+# Hints
+
+## 1. Representing the Date
+
+- The core class for representing dates without time in Java is [`java.time.LocalDate`][localdate-docs].
+- You'll need to work with the `year` and `month` provided in the constructor.
+- The target day of the week is conveniently provided directly as a [`java.time.DayOfWeek`][dayofweek-docs] enum instance in the `day()` method.
+
+## 2. Starting Point for Adjustments
+
+- `LocalDate` objects are immutable. To find the target meetup date, you typically start with *any* date within the target `month` and `year` and then apply an "adjuster" to find the correct day.
+- For example, you could create a `LocalDate` for the 1st of the month: `LocalDate.of(year, month, 1)`.
+- For "Teenth" calculations, starting from the 13th of the month (`LocalDate.of(year, month, 13)`) can be helpful.
+
+## 3. Adjusting Dates with `TemporalAdjusters`
+
+- The key to solving this idiomatically is the [`java.time.temporal.TemporalAdjusters`][temporaladjusters-docs] utility class (note the plural 's'). It provides static methods that return [`TemporalAdjuster`][temporaladjuster-docs] instances for common date calculations.
+- You apply an adjuster to a `LocalDate` using its `.with()` method: `someLocalDate.with(TemporalAdjusters.someAdjuster(...))`
+- This returns a *new* `LocalDate` instance with the adjustment applied.
+
+## 4. Handling "Ordinal" Schedules (FIRST, SECOND, THIRD, FOURTH)
+
+- Look for a method in `TemporalAdjusters` that allows you to find a date based on its ordinal position (1st, 2nd, 3rd, 4th) and its `DayOfWeek` within the month.
+- You might need to convert the `MeetupSchedule` enum values (`FIRST`, `SECOND`, etc.) into the corresponding integer ordinals (1, 2, 3, 4) to use this adjuster.
+
+## 5. Handling the "LAST" Schedule
+
+- `TemporalAdjusters` provides a specific adjuster to find the *last* occurrence of a given `DayOfWeek` within the month.
+
+## 6. Handling the "TEENTH" Schedule
+
+- "Teenth" days run from the 13th to the 19th of the month.
+- Consider starting from the 13th day of the month (`LocalDate.of(year, month, 13)`).
+- Look for an adjuster in `TemporalAdjusters` that finds the *next* occurrence of the target `DayOfWeek`, potentially *including* the date you're adjusting if it already matches the target day of the week.
+
+## 7. Selecting the Right Adjuster
+
+- Use the input `schedule` (which is a `MeetupSchedule` enum value) to determine which specific `TemporalAdjuster` method to use. A `switch` statement or `if-else if` chain on the `schedule` value is a common approach.
+
+[localdate-docs]: https://docs.oracle.com/javase/8/docs/api/java/time/LocalDate.html
+[dayofweek-docs]: https://docs.oracle.com/javase/8/docs/api/java/time/DayOfWeek.html
+[temporaladjuster-docs]: https://docs.oracle.com/javase/8/docs/api/java/time/temporal/TemporalAdjuster.html
+[temporaladjusters-docs]: https://docs.oracle.com/javase/8/docs/api/java/time/temporal/TemporalAdjusters.html

exercises/concept/meetup/.docs/introduction.md.tpl

@@ -0,0 +1,3 @@
+# Introduction
+
+%{concept:strings}