[FIX,INTERNAL] Fixing dox system when using <code>.

The internal representation of @code was as a <code> node.  This is now done
as <dox:code> which allows to distinguish both without @htmlonly.
[DOC] First full version of language_entities.dox

Providing each part of language_entities.dox with some documentation.
We might need to extend this, but for now it should be OK.
Merge pull request #14 from holtgrewe/feature/language_entities_doc

Fixing language_entities.dox