Provide two-factor authentication workflow guide

[riverrun]

There is an Phauxth.Otp plug which provides a one-time password check. However, there is currently no guidance on how to integrate this into a web application. Considering that many security issues arise from implementation errors, there needs to be more information about implementation in the wiki and / or documentation.

[riverrun]

There is no longer an Otp plug, but I am keeping this issue open, so that we can add information about two-factor authentication to the wiki. We can also add the information to the documentation for one_time_pass_ecto.