Documtation to support migration to npm

[bsutton]

The vaadin migration guide (bower to npm) mentions a number of maven/pom.xml changes.

https://vaadin.com/tutorials/migrate-from-bower-to-npm-for-vaadin-14+

Can you provide documentation on what the equivalent gradle changes are:

Specifically the changes mentioned are:

  <groupId>com.vaadin</groupId>
    <artifactId>vaadin-maven-plugin</artifactId>
    <version>${vaadin.version}</version>
    <executions>
      <execution>
        <goals>
          <goal>prepare-frontend</goal>
          <goal>build-frontend</goal>
        </goals>
      </execution>
    </executions>
  </plugin>