Clean readme & move doc

[MaicolBen]

Summary

The idea of this PR is cleaning the long and confusing README, and move a lot of information to a doc section. The docs can be accessed through gitbook, you can search and see the information organized in pages there.

Also, I moved the wiki to the docs as FAQ, so can review the doc changes through PR requests and it's in one place, so the wiki will disappear.

Preview

Issues resolved

Resolves #1114
Resolves #745
Resolves #604
Resolves #75
Resolves #1069

About #604, I tried to cover all the questions/doubts in the existing wiki, there is an alternative flow asked there, that #1072 will cover.
About #75, it's covered in the "Testing with Rspec" section

Notes:

• The gitbook is pointed to my subdomain, but once this PR is merged, we'll change it to Lynn's account
• After this PR is merged, I will point the gitbook doc in https://stackoverflow.com/tags/devise-token-auth/info

[zachfeldman]

This is amazing @MaicolBen !

[dks17]

omniauth.md
Is the link of demo app correct?

For example, the demo app uses the default omniauth_prefix setting /omniauth, so the "Authorization callback URL" for github must be set to "http://devise-token-auth-demo.herokuapp.com**/omniauth**/github/callback".

The demo app looks not working. Is it OK?

[MaicolBen]

@dks17 @lynndylanhurley That's an example, isn't it?

Also, the Angular 2 demo is down, should I remove the link? Or open a new ticket for it?

[dks17]

@MaicolBen I don't know whose the demo this is.
I created issue Live demo is not working.

[zachfeldman]

@MaicolBen I think we should merge this, then file further issues for other outstanding README problems (like possibly non-working demos). Sound good?

[MaicolBen]

Sounds good! I think we need @lynndylanhurley's approval, and we are good to go

[lynndylanhurley]

Looks good, thanks @MaicolBen!!

[neroniaky]

@dks17 I updated the demo to use stackblitz: https://stackblitz.com/github/neroniaky/angular-token
Gonna see how it works and create a PR later.