[docs] migrate to another docs service

[resir014]

trafficstars

Since GitBook has made a lot of updates and severely limiting our free plan, we should consider moving to another documentation provider.

Docz seems like a good thing to migrate to, we can import our existing Markdown docs quite easily, and we can easily host it on Github Pages!

[webbertakken]

Since this is an open source project, you can get GitBook for free.