doc: maven-settings.xml for Github Actions

[nealeu]

After a little digging as to how things run, it's perhaps worth documenting that maven-settings.xml needs to match the one that Github Actions installs during actions/setup-java

  <servers>
    <server>
      <id>github</id>
      <username>${env.GITHUB_ACTOR}</username>
      <password>${env.GITHUB_TOKEN}</password>
    </server>
  </servers>