Move Doxia based modules to contrib

The following rendering modules are based on Doxia:

xwiki-rendering-syntax-doxia
xwiki-rendering-syntax-apt
xwiki-rendering-syntax-docbook
xwiki-rendering-syntax-twiki

None of them are packaged in XWiki Standard and I doubt any are really used much in practice (or ever were to be honest). Doxia 2 started to do some clean and removed support for docbook and twiki and I think we should start to do the same thing.

See https://forum.xwiki.org/t/move-all-doxia-based-modules-to-contrib/15637.