Add optional marketing automation capabilities

[virgile-dev]

Feature Request

Is your feature request related to a problem or unsupported use case? Please describe.
As Docs instance manager I might need to add users into a mailing list so I can let them know when there is a new version or run onboarding sequences so that it's easier for them to get to know the tool

Describe the solution you'd like
Much like @lebaudantoine did on Meet, we could add a marketing.py to our backend services. Which could be called upon signup so that we add the new user to the contact list.
Of course not all Docs instance admin will use the same mailing service or want to use this so we can create an env variable and a Django Setting to configure the mailing service.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Discovery, Documentation, Adoption, Migration Strategy
If you can, explain how users will be able to use this and possibly write out some documentation (if applicable).
Maybe add a screenshot or design?

Do you want to work on it through a Pull Request?