Improve the document of the XWiki Backend

https://cristal.xwiki.org/xwiki/bin/view/Backends/XWiki/ is difficult to read.

Ideas

• Maintain a "recommended install" section with the nominal cases, and move the specific cases (e.g., for a given version) to separate sections
• Mention the existence of a Cristal Integration extension for easier install
• Show more explicitly existing limitations