Use StILi itself as its own Maven site skin (dogfooding)

Use StILi itself as its own Maven site skin (dogfooding)

site.xml now references ${project.groupId}:${project.artifactId}:${project.version}
instead of maven-fluido-skin. Removes the now-unused maven-fluido-skin.version
property from pom.xml.

Build requires ./mvnw clean install site so the skin is installed to the
local repository before the site phase resolves it as a dependency.