Update docs for how to set up SMTP/email access in Sourcegraph

Created by: dadlerj

Requested by https://app.hubspot.com/contacts/2762526/company/557691596

A user asked how to configure SMTP on their instance if they use a public (IP restricted) SMTP server.

Our docs are limited (https://docs.sourcegraph.com/admin/config/site_config#email-smtp) and don't cover which fields are required or what options for the "authenication" setting exist.