Add examples in docs for authentication and sending emails

[colinkiama]

I think that it would make it easier for people to get started with the API.

They would be able to get something working much faster.

[dns2utf8]

Hi @colinkiama

I implemented a service with a modified version of this crate:
https://gitlab.com/dns2utf8/one_fa_auth

Hope this helps.
Cheers,
Stefan