Revisit Sitemap plugin

As investigated in ESSENTIALS-211, the certified sitemap plugin provides a number of ways to generate a sitemap. The provided solutions are either too complex and ill-performing, or (too?) simple.

Currently, the sitemap plugin uses the simple approach of producing a sitemap entry per menu item. This can give users a false feeling of comfort.

We should therefore evaluate the value of this Essentials (and certified! forge) plugin, and determine if we want to keep it in Essentials, and if we need to improve it. Improvements would probably first target the CMS/site plugin part, and only then the Essentials dashboard plugin.