Contributing to the Gallery - needs updating to new gallery

[robertwray]

The steps and screenshots in the "Contributing to the Gallery" doc page are out of date WRT the new orchard gallery and need to be updated to reflect:

1. The steps that have changed
2. Some of the steps/options outlined in the documentation are no longer present (e.g. "Registering a Package ID in Advance")
3. The fact that the user interface is now different (the styling of the gallery is different)