Add site.xml to project archetypes

[DavidWhitlock]

As part of fixing the Maven site for the projects (https://github.com/DavidWhitlock/PortlandStateJavaSummer2022/pull/1), I realized that, with the "fluido" skin, all projects must have a site.xml file in order for their site's index.html pages to contain any links.

So, I'll have to add site.xml files to the project archetypes.

[DavidWhitlock]

And we probably want to add this to the top-level POM:

      <plugin>
        <artifactId>maven-scm-publish-plugin</artifactId>
        <version>3.1.0</version>
        <configuration>
          <scmBranch>gh-pages</scmBranch>
          <tryUpdate>true</tryUpdate>
          <addUniqueDirectory>true</addUniqueDirectory>
        </configuration>
      </plugin>