Add mailing list subscription button (via Revue)
Add mailing list subscription button.
I briefly evaluated Revue and Mailchimp. Revue seemed more aesthetic and easy to use. Also, see https://www.getrevue.co/profile/dagster vs http://eepurl.com/hnYH7r. It's easy to switch between providers later on so just going to go with Revue
Open to using a different icon
Test Plan: previewed docs locally
Reviewed By: yuhan
Differential Revision: https://dagster.phacility.com/D5986