Create a common documentation folder

### What needs to get done

Similar to craft-parts, we should have some documentation that can be reused in applications.

### Why it needs to get done

Things like environment variables (#412) should be able to be passed to the application.

Start by reviewing how craft-parts organizes reusable documentation and reading issue #412 for the environment-variable context. Define what documentation belongs in the shared folder and how applications will consume it; done means the common structure and initial reusable documentation are clearly established.

Written by the indexing model from the issue text.