Add full API guide with extensions like PowEmailConfirmation and PowResetPassword

[danschultzer]

@mreishus has written two guides that details how to implement the extensions in API setup:

https://github.com/mreishus/notebook-pub/blob/master/programming/phoenix/pow/adding-email-confirmation-to-an-api.md

https://github.com/mreishus/notebook-pub/blob/master/programming/phoenix/pow/adding-reset-password-to-an-api.md

There already is a basic API guide in the docs, but it's written to be as flexible as possible so it can work for most setups. Very few assumptions are made. I think that the above guides can be integrated in a full A-Z API guide for e.g. SPA sites where everything is laid out.

[christophweegen]

Are there any updates on this topic planned in the foreseeable future?

Let me know if I can help 👍