automatic documentation build

[wilzbach]

Like https://docs.rs/ (e.g. here) the documentation could be automatically built by basically running running dub build -b ddox and storing the output somewhere.

While of course this doesn't have to be within the dub-registry, I though that this is the best place where people interested in improving code.dlang.org would look.

There is one project that goes into the direction: http://dpldocs.info, but only builds the docs for Phobos, druntime + arsd (it's based on arsdox).

[MartinNowak]

Done with the dpldocs link in #302