Refactor README for configuration

[Khwaj7]

Refactor README to move the configuration to the correct place

Thanks for this useful project!

Summary

I tried setting the tag in the part, but this part only accepts children.

I think the should be in the corresponding , except if it's common to multiple so we can set it as a child of tag.

Tests and Documentation

Here is my working pom:

<plugin>
      <groupId>com.github.eirslett</groupId>
      <artifactId>frontend-maven-plugin</artifactId>
      <version>1.15.4</version>
      <executions>
            <execution>
                  <id>install-node-and-corepack</id>
                  <goals>
                        <goal>install-node-and-corepack</goal>
                        <goal>yarn</goal>
                  </goals>
                  <!-- optional: default phase is "generate-resources" -->
                  <phase>generate-resources</phase>
                  <configuration>
                        <arguments>-v</arguments>
                        <nodeVersion>v22.4.1</nodeVersion>
                        <corepackVersion>v0.34.3</corepackVersion>
                        <yarnVersion>v4.11.0</yarnVersion>
                  </configuration>
            </execution>
      </executions>
</plugin>

Here is the pom if I follow the actual README. It generates an error as the tag can't be placed here:

<plugin>
    <execution>
        <id>install-node-and-corepack</id>
        <goals>
            <goal>install-node-and-corepack</goal>
        </goals>
    </execution>
    <configuration>
        <nodeVersion>v20.12.2</nodeVersion>

        <!-- Optional - only needed if Node <16.9, or if you need to use a version different
             from the one packaged with Node -->
        <corepackVersion>v0.25.2</corepackVersion>

        <!-- optional: where to download node from. Defaults to https://nodejs.org/dist/ -->
        <nodeDownloadRoot>http://myproxy.example.org/nodejs/</nodeDownloadRoot>
        <!-- optional: where to download corepack from. Defaults to https://registry.npmjs.org/corepack/-/ -->
        <corepackDownloadRoot>http://myproxy.example.org/corepack/</corepackDownloadRoot>
    </configuration>
</plugin>

[eirslett]

I think the example is wrong in the sense that it's lacking <executions> outside of <execution>?
The point of keeping the configuration outside, is that if you have several executions of the plugin, they should all share the same node/npm whatever common configuration!