Improve documentation for single compared to multiple application instance creation

[eric-murray]

Problem description
Single application instances are created using POST /app-instances
Multiple application instances are created using POST /deployments

The documentation does not make this distinction clear and needs to be improved

The need for POST /app-instances at all is not clear when a single instance can be deployed using POST /deployments. Why allow application instances to exist that are not part of a deployment?

It is not documented what happens when DELETE /app-instances is used for an application instance that is part of a deployment. Can the API consumer expect that the deployment is properly updated?

Expected action
Improve the documentation explaining clearly the different use cases:

• Deploy a single instance of an application using POST /app-instances
• Deploy multiple instances of an application using POST /deployments

Explain the life cycle of an application instance that is not part of a deployment, and how that compares to an application that is part of a deployment.

Review the need to keep the POST /app-instances in the design, and remove it if the functionality can all be done using POST /deployments

Additional context
None