Revamp the docs

[Sobak]

Hopefully this pull request turned out not to be too long to review... I went through the docs line by line and tried to improve while paying the attention to the following:

• bringing internal consistency between examples in regard to how is User model namespaced, whether ::class is used or not, their indendation level etc...
• changing code style to that used and promoted by Laravel (PSR-2)
• fixing formatting and minor errors (like invalid config file path) here and there

I also listed differiences between this and the original package (taken from #55), updated tables of contents for new aspects discussed within advanced docs and clarified on how enum type works internally (separate commit).

@rebing I'd be glad if you could find a time to review and eventually merge this changeset. I have a couple more small ideas on top of my head (like revamping package's generators provided in make:graphql:* and they would fit the best with more consistent docs promoting one particular style and convention.

[rebing]

Looks good, nice job! Thanks for the support