Add new maven goal to collect config docs when creating distribution

[daneshk]

With this change, in order to collect config-docs from product distribution, we need to add below plugin configuration to distribution pom.xml after carbon-feature-plugin.

            <plugin>
                <groupId>org.wso2.carbon.config</groupId>
                <artifactId>org.wso2.carbon.config.maven.plugin</artifactId>
                <version>2.0.6-SNAPSHOT</version>
                <executions>
                    <execution>
                        <phase>package</phase>
                        <goals>
                            <goal>collect-docs</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>