Add more guidance on manifest/space best practices

[afeld]

I wrote this in Slack:

my recommendation these days:

• have a base manifest that specifies the common configuration that will look the same across all spaces. this should include things like the name(s) for the application(s), the same service instance names, etc.
• make a manifest per environment that inherits from the base one, specifying the environment-specific things, e.g. routes/hostnames, feature flags, etc.

See that conversation for more background. Could have sworn we had an issue/pull request around this already...apologies if this is a duplicate.

/cc @dogweather @LinuxBozo @cmc333333

[afeld]

Related: the relatively-new org creation script now creates dev, staging, and prod spaces by default, which should nudge people into structuring their deployments that way.