Support multiple environments for DEV/TEST/LIVE (other?)

I rely on other people to specify their 'config sync' policy / best practices. So I'll rebrand this a task.

People must be doing this. But so far, I don't remember anyone really commenting on it.

Are they using config_ignore? If so, what are they ignoring? Just the entity ID (provided the certs are in files)?

Do they want to use multiple IdP configs for this? ✨ Multiple IDP Support Needs work

When there's a somewhat authoritative reply from someone, we can determine if this needs documentation, or other work.