Add Maven Site goal back to GitHub action

[philsmart]

Running the Maven Site goal on ukf-members generates the following error:

Could not transfer artifact net.shibboleth:parent-v3:xml:site_en:9 from/to maven-default-http-blocker (http://0.0.0.0/): Blocked mirror for repositories: [central (http://repo1.maven.org/maven2, default, releases)]
  net.shibboleth:parent-v3:xml:9

This seems to be because the parent project (net.shibboleth:parent-v3:9) does not have a site descriptor (site_en.xml or site.xml) in the .m2 repository when the ukf-members site is being generated.

One fix is to add a site descriptor to the parent project. However, as we can not alter the plugin configuration in the already-released parent project, I will need to find other ways to allow site generation.

See also: https://stackoverflow.com/questions/23597456/how-to-avoid-checking-parent-project-in-maven-site-plugin
See also: https://tcollignon.github.io/2016/10/24/Unable-to-locate-site-descriptor-maven-site-plugin-pro
See also: #7

[iay]

I think we can defer working on this until we see whether the problem gets simpler when we move to the Java 17 parent.

Even then, I'm much less convinced of the value of Maven sites now than I was when I put this project together, and if the barrier is too high I might be tempted to say let's drop it, with the possible exception of the API documentation.