Zero downtime deployment guidance

Currently, in the docs we have guidance on how to setup a server on DigitalOcean (mostly transferable to other virtual server providers), on Heroku and on any provider that supports Docker. These are 3 different deployment types that we provide official help for.

But in a production environment, it is also important to make sure that while we're deploying a new version of a Vapor app to the server, that the servers are not down (for more than a few seconds). This is called "Zero Downtime Deployment". Currently, the docs have basically no information on the downtime at all. There might even be already some things in place (e.g. by using SuperVisor/Nginx/Docker features) which help with that, but I can't find clear information on it and I think it's important.

Otherwise especially beginners will probably not be aware that while they're deploying a new version – ans especially if something goes wrong during the deployment – their APIs or websites might be offline for a while.

Any inputs, ideas or links about this topic related to Vapor?