XWIKI-13946: The headings from the installation instructions are not displayed in the table of contents
* Include the extra installation instructions as is (un-rendered) in order for the ToC macro to be able to find their headings.

Done

Fixed

* The $appDescriptorClassName variable needs to be defined globally (used to be the case but I lost it during a refactoring).
* The current doc is the template provider and not the class (it used to be the class but we moved this Velocity macro).