Update documentation source from keystonejs-site to keystone/docs

[htor]

documentation is is currently spread out all over the place and is often outdated. after what i gathered @Noviny started collecting all documentation (keystonejs/keystone#4181) into a new website hosted here: http://keystonejs.netlify.com/documentation.

i think that's a good solution to keeping things organised and clean, instead of hiding readme-s under each directory with the source code.
i want to help with documenting, so i wonder: what is the current situation? what needs to be done? do we merge this site into the main site (http://keystonejs.com/) when documentation is more ready or is it meant to live by itself on a different domain?

this is the first commit of cleaning something up, and i hope to commit more to this branch, once i know a bit more about the help needed.

[stennie]

@htor Thank you for taking the initiative to create this pull request! The documentation definitely needs some revamping and the Contributor Guide is a great starting point to help onboard new contributors.

There are more extensive updates needed, so I'm going to close this PR in favour of a more complete update of the Contributing Guide for Keystone 4 per #4653.

Regards,
Stennie