Document the ApplicationPropertiesCustomizer extension point

Describe in the configuration guide how application properties can be
contributed to the generated project, including how to target another
source set or Spring profile through sections, with a runnable example.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Signed-off-by: Denis A. Altoé Falqueto <denisfalqueto@gmail.com>

Author: denisfalqueto

initializr-docs/src/main/asciidoc/configuration-guide.adoc

@@ -215,6 +215,21 @@ requires you to add a certain plugin to the build, you can provide a `BuildCusto
 that adds the plugin and the customizer will be called according to the order specified on
 it.
 
+Similarly, an `ApplicationPropertiesCustomizer` can be used to contribute properties to
+the application configuration file of the generated project, that is
+`application.properties` or `application.yaml` depending on the requested
+`ConfigurationFileFormat`. Properties added directly to the `ApplicationProperties`
+argument are written to the configuration file of the main source set and the default
+profile. Properties for another source set or Spring profile can be added to the matching
+`section`, and a `src/{sourceSet}/resources/application[-{profile}].{properties|yaml}`
+file, resolved against the project's build system, is generated for every section that
+contains properties, as shown in the following example:
+
+[source,java,indent=0]
+----
+include::{code-examples}/doc/generator/project/ProjectCustomizationExamples.java[tag=application-properties-customizer]
+----
+
 
 
 [[initializr-generator-spring-requirements]]

initializr-docs/src/main/java/io/spring/initializr/doc/generator/project/ProjectCustomizationExamples.java

@@ -25,6 +25,8 @@
 import io.spring.initializr.generator.project.ProjectDescriptionCustomizer;
 import io.spring.initializr.generator.project.ProjectDescriptionField;
 import io.spring.initializr.generator.spring.build.BuildCustomizer;
+import io.spring.initializr.generator.spring.properties.ApplicationPropertiesCustomizer;
+import io.spring.initializr.generator.spring.properties.SourceSet;
 
 import org.springframework.context.annotation.Bean;
 
@@ -69,4 +71,18 @@ private boolean isBelowJava17(String jvmVersion) {
 	}
 	// end::jvm-version-change-reason[]
 
+	// tag::application-properties-customizer[]
+	@Bean
+	public ApplicationPropertiesCustomizer applicationPropertiesCustomizer() {
+		return (properties) -> {
+			// src/main/resources/application.properties (or .yaml)
+			properties.add("spring.application.name", "acme");
+			// src/test/resources/application.properties
+			properties.section(SourceSet.TEST).add("spring.datasource.url", "jdbc:h2:mem:acme");
+			// src/main/resources/application-dev.properties
+			properties.section(SourceSet.MAIN, "dev").add("logging.level.root", "DEBUG");
+		};
+	}
+	// end::application-properties-customizer[]
+
 }