showcase: create package

[dsebastien]

Create the showcase app. It will host:

• be a Stark app!
• our showcase for stark features/components
• our developer guide
• links to api docs

Tasks:

• create the empty showcase based on the (cleaned up!) starter
• adapt the contributing guidelines to indicate that any feature added/changed/... must be explained/shown in the showcase application and thus that the showcase (in addition to the starter) must always be kept in sync with the latest Stark version)
• (later) integrate existing wiki docs to the showcase