Generate empty spring-boot-docs jar

Update spring-boot-docs so that an empty jar file is produced. The
code is only needed for documentation includes and if included in the
deployed jar seems to prevent promotion to Maven Central.

Fixes gh-6495

Author: philwebb

spring-boot-docs/pom.xml

@@ -731,6 +731,19 @@
 			<optional>true</optional>
 		</dependency>
 	</dependencies>
+	<build>
+		<plugins>
+			<plugin>
+				<groupId>org.apache.maven.plugins</groupId>
+				<artifactId>maven-jar-plugin</artifactId>
+				<configuration>
+				<excludes>
+					<exclude>**</exclude>
+				</excludes>
+				</configuration>
+			</plugin>
+		</plugins>
+	</build>
 	<profiles>
 		<profile>
 			<id>full</id>