Model: Design documentation for AppConfiguration model

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What would you like to be added?

Design documentation for the new AppConfiguration model which will eventually phase out the current Server model. AppConfiguration model aims to provide more comprehensive support and better developer experience when describing an application running in Kubernetes and clouds. The core principles for the new AppConfiguration model is developer first, application-centric and platform agnostic.
Both models should be supported for the foreseeable future.

Why is this needed?

The current Server model is outdated in terms of the resources it can support and insufficient in meeting our designed use cases for kusion for longer term.
The new AppConfiguration aims to solve that problem via its core design principles: developer first, application-centric and platform agnostic.

Acceptance Criteria

A design document for AppConfiguration is produced and stored in the KusionStack org with a sample.