Abstract DDoc output for better document generation

[Tolga Cakiroglu via Digitalmars-d]

After having some experience with documentation generation from
dmd command like, and seeing outputs, I wasn't satisfied with it.
Other alternatives as Doxygen is available, though it parses
codes itself that is not based on what version of DMD is
installed on system.

Since the structure of DDoc is already defined, instead of
generating HTML output, I would propose to generate an XML file
that contains meta data about codes and comments which can be
used by 3rd party documentation generator programmes to generate
HTML files. This way DDoc definitions can be simplified, better
looking documents can be generated much easily.