Initial restructure of docs for better organization

To help better organize the documentation, especially as more is added (and it should be), I'm suggesting we restructure the `Documentation` folder in the filesystem to group docs into top level categories.

This will require some rewrites to the `make docu` CLI command to support, but that's easy enough I'd say.   Also up for discussion would be the URL routing here.  Instead of http://issues.joomla.org/devdox/dox-app-debug, perhaps a URL like http://issues.joomla.org/docs/applications/debug (which matches the file structure still) could be used.

Also, I started a little bit on standardizing English use.  Our default language is en-GB, might as well get the docs right with that too.