Add maven plugin documentation

This commit adds the generated site for the maven plugin alongside
the developer guide and javadoc. The maven plugin is available in
the "/maven-plugin" context.

The advanced information described in the developer guide have
been migrated to the plugin site as most the information is taken
from the code itself, which avoids duplication.

Fixes #749

spring-boot-docs/pom.xml

@@ -223,6 +223,31 @@
 							</execution>
 						</executions>
 					</plugin>
+					<plugin>
+						<groupId>org.apache.maven.plugins</groupId>
+						<artifactId>maven-dependency-plugin</artifactId>
+						<executions>
+							<execution>
+								<id>unpack-maven-plugin</id>
+								<phase>generate-resources</phase>
+								<goals>
+									<goal>unpack</goal>
+								</goals>
+								<configuration>
+									<artifactItems>
+										<artifactItem>
+											<groupId>org.springframework.boot</groupId>
+											<artifactId>spring-boot-maven-plugin</artifactId>
+											<version>${project.version}</version>
+											<classifier>site</classifier>
+											<type>zip</type>
+											<outputDirectory>${project.build.directory}/contents</outputDirectory>
+										</artifactItem>
+									</artifactItems>
+								</configuration>
+							</execution>
+						</executions>
+					</plugin>
 					<plugin>
 						<groupId>org.apache.maven.plugins</groupId>
 						<artifactId>maven-antrun-plugin</artifactId>