ESC: Ensure that the use cases for the SDK are clear

[jkodroff]

A customer got confused and ended up using the ESC SDK within their Pulumi IaC programs. We should make it clear that consuming environments via the ESC SDK is for workloads, usually at runtime, while Pulumi IaC programs should be consuming ESC environments via config.

Probably want to disambiguate here: https://www.pulumi.com/docs/esc/development/

We will need to check for other places in the ESC docs where a user might need clarification.