chore(docs): add example project (#291)

modern-treasury-java-example/build.gradle.kts

@@ -1,12 +1,18 @@
 plugins {
-    id("application")
-    id("modern-treasury.java")
+    id("modern-treasury.kotlin")
+    id("java")
+    application
 }
 
 dependencies {
     implementation(project(":modern-treasury-java"))
 }
 
+tasks.withType<JavaCompile>().configureEach {
+    // Allow using more modern APIs, like `List.of` and `Map.of`, in examples.
+    options.release.set(9)
+}
+
 application {
-    mainClass.set("com.moderntreasury.example.Main")
+    mainClass = "com.moderntreasury.api.example.Main"
 }