docs: add class diagram for core entities in C4 architecture

- Created `class_diagram_core.drawio` to visualize core domain entities (`Veranstaltung`, `Platz`, `Turnier`, etc.).
- Included relationships and associations between entities such as 1:N and reference mappings.
- Structured diagram to align with Clean Architecture principles for better domain comprehension.

core/core-domain/src/commonMain/kotlin/at/mocode/core/domain/model/Enums.kt

@@ -524,3 +524,15 @@ enum class PlatzTypE {
   /** Sonstige Fläche */
   SONSTIGE
 }
+
+/**
+ * Reglement, dem das Turnier unterliegt.
+ */
+@Serializable
+enum class ReglementE {
+  /** Nationales Reglement (ÖTO) */
+  OETO,
+
+  /** Internationales Reglement (FEI) */
+  FEI
+}