Add rel="nofollow" to xwiki links that are not pointing to user content

Similarly to XWIKI-15386, we still have many places where we need to make sure links not pointing to user content (i.e. UI actions) are not indexed by search bot crawlers and do not pollute search results from XWiki instances.

The most common pattern I noticed was related to "xpage=zyx" links, but I am sure others exist.

This can also be handled on the robots.txt side, by the person installing XWiki, but we should manage it from the product side as much as possible.