Update documentation to mention execution id of repackage goal

This commit updates the documentation to reference the default execution
id of the `repackage` goal when a project uses
`spring-boot-starter-parent`.

Closes gh-14835

Author: snicoll

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/site/apt/examples/custom-layout.apt

@@ -22,6 +22,7 @@
         <version>${project.version}</version>
         <executions>
           <execution>
+            <id>repackage</id>
             <goals>
               <goal>repackage</goal>
             </goals>

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/site/apt/examples/repackage-classifier.apt.vm

@@ -29,6 +29,7 @@
         <version>${project.version}</version>
         <executions>
           <execution>
+            <id>repackage</id>
             <goals>
               <goal>repackage</goal>
             </goals>
@@ -45,6 +46,38 @@
   </build>
   ...
 </project>
+---
+
+  If you are using `spring-boot-starter-parent`, the `repackage` goal is executed
+  automatically in an execution with id `repackage`. In that setup, only the configuration
+  should be specified as shown in the following example:
+
+---
+<project>
+  ...
+  <build>
+    ...
+    <plugins>
+      ...
+      <plugin>
+        <groupId>${project.groupId}</groupId>
+        <artifactId>${project.artifactId}</artifactId>
+        <executions>
+          <execution>
+            <id>repackage</id>
+            <configuration>
+              <classifier>exec</classifier>
+            </configuration>
+          </execution>
+        </executions>
+        ...
+      </plugin>
+      ...
+    </plugins>
+    ...
+  </build>
+  ...
+</project>
 ---
 
   This configuration will generate two artifacts: the original one and the repackaged
@@ -84,6 +117,7 @@
         <version>${project.version}</version>
         <executions>
           <execution>
+            <id>repackage</id>
             <goals>
               <goal>repackage</goal>
             </goals>
@@ -106,5 +140,48 @@
   same phase, it is important that the jar plugin is defined first (so that it runs before
   the repackage goal).
 
+  Again, if you are using `spring-boot-starter-parent`, this can be simplified as follows:
+
+---
+<project>
+  ...
+  <build>
+    ...
+    <plugins>
+      ...
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-jar-plugin</artifactId>
+        <executions>
+          <execution>
+            <id>default-jar</id>
+            <configuration>
+              <classifier>task</classifier>
+            </configuration>
+          </execution>
+        </executions>
+    </plugin>
+      <plugin>
+        <groupId>${project.groupId}</groupId>
+        <artifactId>${project.artifactId}</artifactId>
+        <executions>
+          <execution>
+            <id>repackage</id>
+            <configuration>
+              <classifier>task</classifier>
+            </configuration>
+          </execution>
+        </executions>
+        ...
+      </plugin>
+      ...
+    </plugins>
+    ...
+  </build>
+  ...
+</project>
+---
+
+