Fully remove the specific Ubuntu guides

[QuintenQVD0]

• Remove he ubuntu2004 and ubuntu1804 files so the search function does not list them anymore, they were already removed from the sidebar
• Fix a forgoten reference to Ubuntu 18.04 been supported on the wings install page
• Remove in the add_node node file the reference to the Ubuntu guide as stock is now Ubuntu and point to the debian guide as the debian9 guide does not exist anymore