chore: restructure package to match score-compose layout

[astromechza]

We've received some feedback that the score-k8s layout is confusing when comparing it to the other score-compose reference example and that it would be easier to understand if it were layed out more similarly.

This PR does some of this migration and clean up. No impact to the runtime is expected.

1. Setup internal/command package with the cobra commands
2. Move the main.go to cmd/score-k8s/main.go
3. Move some images and create the empty docs directory

[mathieu-benoit]

LGTM, more consistency with score-compose repo, love it!

[delca85]

👍