Update mail server configuration section

[javierm]

References

• Closes Update email configuration documentation, please. #95
• Closes Update documentation for email in development #64
• Continues pull request Update dev_mailserver.md #101
• We changed the way email servers are configured in pull request Define SMTP settings in secrets file consuldemocracy#3695

Background

We don't recommend using a mail server in the development environment; in that environment, mails are opened in the browser. Even so, the documentation about configuring an email server referenced the development environment.

Objectives

• Update the mail server configuration so it's clear that it applies to production environments
• Update the mail server configuration so it uses the secrets file instead of the environment file