Add a ddox based HTML documentation generator for dlang.org

Adds a small tool that builds HTML documentation with a single page per entity using [ddox](https://github.com/rejectedsoftware/ddox). It is built using [dub](http://github.com/rejectedsoftware/dub), which is available as a precompiled binary at <http://registry.vibed.org/download>.

Building:
```
cd dpl-docs
dub build
```

The tool is then used to generate the documentation from DMD's JSON output (separate pull request in the dpl.org repository).