Use DocC if open-sourced

[mikolasstuchlik]

An official Apple account mentioned, that DocC might get open-sourced someday.

If it does, we should use it instead of 3rd party library.

We should also update gir2swift in order to get advantage of the new grammar.

https://developer.apple.com/forums/thread/682023

[rhx]

Yes, this is definitely something that would be nice. Apart from not being open source (yet), another potential stumbling block is that DocC is not compatible with GitHub Pages at the moment.

[itjustcrashed]

@rhx https://www.swift.org/blog/swift-docc/
:) its open now

[rhx]

Yes, I'm transitioning things over on the development branch (not yet ready to merge, though).