Add getExternalDocumentationLink

Author: psainics

postgresql-plugin/src/main/java/io/cdap/plugin/postgres/PostgresSource.java

@@ -77,6 +77,11 @@ protected Class<? extends DBWritable> getDBRecordType() {
     return PostgresDBRecord.class;
   }
 
+  @Override
+  protected String getExternalDocumentationLink() {
+    return DBUtils.POSTGRES_SUPPORTED_DOC_URL;
+  }
+
   @Override
   protected LineageRecorder getLineageRecorder(BatchSourceContext context) {
     String fqn = DBUtils.constructFQN("postgres",