New documentation engine and styling

[daffl]

The documentation at crow.docs.feathersjs.com is currently being migrated to Vuepress a static documentation generator also used by VueJS which covers pretty much all the use cases also needed for the Feathers documentation.

The change is necessary because GitBook has transitioned to a hosted service and is no longer maintaining the open source version of the generator which is currently used for the Feathers documentation.

It already looks great but it would be great to get some help making the styles a little more Feathers specific.