Add design document for the Meetup concept exercise

Introduces learning objectives, out-of-scope topics, concepts, prerequisites, and analyzer rules for the exercise. Clarifies focus on `TemporalAdjusters` and proper usage of `java.time` API while providing guidance for evaluations in the analyzer.

Author: Jegors Cemisovs

exercises/concept/meetup/.meta/design.md

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+# Design
+
+## Learning objectives
+
+- Know about the `java.time.temporal.TemporalAdjuster` interface and its purpose.
+- Know about the `java.time.temporal.TemporalAdjusters` utility class and how to use its static factory methods.
+- Know how to apply a `TemporalAdjuster` to a `LocalDate` using the `.with()` method.
+- Know how to select and use common predefined adjusters like `TemporalAdjusters.dayOfWeekInMonth()`, `TemporalAdjusters.lastInMonth()`, and `TemporalAdjusters.nextOrSame()`.
+- Understand that `java.time` objects are immutable and adjustment methods return new instances.
+- Know how to use a `java.time.DayOfWeek` enum value.
+- Know how to use a custom `enum` (like `MeetupSchedule`) in conditional logic (e.g., `switch` statement).
+
+## Out of scope
+
+- Creating complex custom `TemporalAdjuster` implementations.
+- Time zones, offsets, `ZonedDateTime`.
+- `LocalDateTime`, `LocalTime`.
+- Durations (`java.time.Duration`) and Periods (`java.time.Period`).
+- Date/time parsing and formatting.
+
+## Concepts
+
+- `temporal-adjusters`: This exercise introduces and focuses on the `TemporalAdjusters` concept.
+- `datetime`: Reinforces basic usage of `LocalDate` and `DayOfWeek`.
+- `enums`: Reinforces usage of enums.
+
+## Prerequisites
+
+- `datetime`: Must know how to create and use basic `java.time.LocalDate` objects and understand `DayOfWeek`.
+- `enums`: Must know how to work with `enum` types, including using them in `switch` statements or accessing properties like `ordinal()`.
+- `classes`: Basic understanding of Java classes and methods.
+- `methods`: Basic understanding of Java methods.
+
+## Analyzer
+
+This exercise could benefit from the following rules in the [analyzer]:
+
+- `essential` (replaces `actionable` if it MUST be fixed): If the solution involves significant manual date calculation logic (e.g., loops checking day of week, complex `plusDays`/`minusDays` sequences) instead of using `TemporalAdjusters`, instruct the student that using the `TemporalAdjusters` class is the idiomatic approach for this exercise. Provide a link to the `TemporalAdjusters` concept documentation.
+- `actionable`: If the student uses `TemporalAdjusters.dayOfWeekInMonth()`, check if they correctly map the `MeetupSchedule` enum ordinal (0-based) to the required 1-based ordinal argument. If not, provide a hint.
+- `actionable`: If the logic for `MeetupSchedule.TEENTH` seems incorrect or overly complex, suggest using `TemporalAdjusters.nextOrSame()` applied to the 13th day of the month.
+- `informative`: If the student uses `TemporalAdjusters` correctly, mention the specific adjusters used (`dayOfWeekInMonth`, `lastInMonth`, `nextOrSame`) as good choices for their respective cases.
+
+If the solution uses `TemporalAdjusters` correctly for all `MeetupSchedule` cases, it should be considered exemplar. Leave a `celebratory` comment to celebrate the success!
+
+[analyzer]: https://github.com/exercism/java-analyzer