Test and document letsencrypt renewals

[UsualSpec]

Currently, we did not test what happens if the Lets encrypt certificate gets renewed automatically. In future, we may need to add a pre and post hook in for certbot to restart mmserver: https://eff-certbot.readthedocs.io/en/stable/using.html#renewing-certificates

Quoting @romain-jacob (private):

It could be that the connection between clients and server gets interrupted after the server renews its certificate. To mitigate that, we should add some pre and post hooks to the cert renewal to stop and restart mmserver