In T396155: Improve verification email, @Urbanecm_WMF wrote new classes for confirm email building. There are further opportunities for improving the code quality of the classes created here, which are tracked within this task.

Acceptance criteria

• Add interface documentation for IConfirmEmailBuilder, ConfirmEmailData and ConfirmEmailContent that describes what it is for and how it should be used
• Consider further generalizing ConfirmEmailConfirm and/or ConfirmEmailSender::sendEmailToRecipient to all emails (as the emailing logic is not specific to confirm email) and fill out appropriate Phabricator tasks