Add MyCustomFormat.kt example to documentation

Closes gh-42594

spring-boot-project/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/features/logging/structured/otherformats/MyCustomFormat.kt

@@ -0,0 +1,12 @@
+package org.springframework.boot.docs.features.logging.structured.otherformats
+
+import ch.qos.logback.classic.spi.ILoggingEvent
+import org.springframework.boot.logging.structured.StructuredLogFormatter
+
+class MyCustomFormat : StructuredLogFormatter<ILoggingEvent> {
+
+	override fun format(event: ILoggingEvent): String {
+		return "time=${event.instant} level=${event.level} message=${event.message}\n"
+	}
+
+}