Document how to use Hubspot for transactional emails for normal system email delivery

Hubspot supports two types of transactional emails - one using their API where the emails are tracked against contacts, and one that's anonymous that doesn't interact with their contact system. I think the best option here would be to use SMTP and the OAuth integration being worked on in ✨ Implement OAuth to support GMail/GSuite accounts Needs review rather than reinventing the wheel.

I think we can change this to a documentation task and show how to use either the Drupal Symfony Mailer module → or the SMTP module → .