Add section on per-branch staging systems

[marcoow]

When talking about pull requests, we should also have a section that recommends setting up a per-branch staging system so changes can be reviewed without having to run the app locally. This should include:

• every PR should have its own preview system with the URL linked in the PR and a well-defined data set
• this is particularly helpful for sharing changes with non-technical people
• this can be hard to set up but if planned from the beginning will be less so, in particular when things are containerized from the beginning
• if setting this up is not possible or there is prohibitive effort, there should be a central preview system that everyone shares (and that is separate from the staging/pre-prod system)