Split up adoc files

Closes gh-23254

spring-boot-project/spring-boot-docs/src/docs/asciidoc/using/structuring-your-code.adoc

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+[[using.structuring-your-code]]
+== Structuring Your Code
+Spring Boot does not require any specific code layout to work.
+However, there are some best practices that help.
+
+
+
+[[using.structuring-your-code.using-the-default-package]]
+=== Using the "`default`" Package
+When a class does not include a `package` declaration, it is considered to be in the "`default package`".
+The use of the "`default package`" is generally discouraged and should be avoided.
+It can cause particular problems for Spring Boot applications that use the `@ComponentScan`, `@ConfigurationPropertiesScan`, `@EntityScan`, or `@SpringBootApplication` annotations, since every class from every jar is read.
+
+TIP: We recommend that you follow Java's recommended package naming conventions and use a reversed domain name (for example, `com.example.project`).
+
+
+
+[[using.structuring-your-code.locating-the-main-class]]
+=== Locating the Main Application Class
+We generally recommend that you locate your main application class in a root package above other classes.
+The <<using#using.using-the-springbootapplication-annotation, `@SpringBootApplication` annotation>> is often placed on your main class, and it implicitly defines a base "`search package`" for certain items.
+For example, if you are writing a JPA application, the package of the `@SpringBootApplication` annotated class is used to search for `@Entity` items.
+Using a root package also allows component scan to apply only on your project.
+
+TIP: If you don't want to use `@SpringBootApplication`, the `@EnableAutoConfiguration` and `@ComponentScan` annotations that it imports defines that behaviour so you can also use those instead.
+
+The following listing shows a typical layout:
+
+[indent=0]
+----
+	com
+	 +- example
+	     +- myapplication
+	         +- Application.java
+	         |
+	         +- customer
+	         |   +- Customer.java
+	         |   +- CustomerController.java
+	         |   +- CustomerService.java
+	         |   +- CustomerRepository.java
+	         |
+	         +- order
+	             +- Order.java
+	             +- OrderController.java
+	             +- OrderService.java
+	             +- OrderRepository.java
+----
+
+The `Application.java` file would declare the `main` method, along with the basic `@SpringBootApplication`, as follows:
+
+[source,java,pending-extract=true,indent=0]
+----
+	package com.example.myapplication;
+
+	import org.springframework.boot.SpringApplication;
+	import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+	@SpringBootApplication
+	public class Application {
+
+		public static void main(String[] args) {
+			SpringApplication.run(Application.class, args);
+		}
+
+	}
+----