Clarify documentation homepage with regards to the future of the plugin and relation to site deployment

New feature, improvement proposal

Currently it is stated in https://github.com/apache/maven-scm-publish-plugin/blob/maven-scm-publish-plugin-3.3.0/src/site/apt/index.apt.vm inside Future Dreams that the plugin should become obsolete but rather part of m-site-p. However there is https://github.com/apache/maven-site-plugin/issues/1129 discussing whether publishing/deployment should rather be separated out from m-site-p.

Also it currently assumes this is only used for publishing site, hower this plugin can publish arbitrary filesystem structures. Although site deployment is probably the most prominent example it should be clarified, that this can be used for other stuff as well.

Start with src/site/apt/index.apt.vm, especially the Future Dreams section and the current description of the plugin's use. Read maven-site-plugin issue 1129 for the deployment discussion, then update the homepage to explain the plugin's future direction and that it can publish arbitrary filesystem structures, not only Maven sites. Done means the documentation is accurate and no longer implies site publishing is its only use.