Add doc goal for scala-maven-plugin (#22734)

Author: lostiniceland

modules/openapi-generator-mill-plugin/pom.xml

@@ -137,6 +137,7 @@
                         <goals>
                             <goal>compile</goal>
                             <goal>testCompile</goal>
+                            <goal>doc-jar</goal>
                         </goals>
                     </execution>
                 </executions>