[Topic] Document how to change the default Reveal images

[brianlagunas]

Description

By default Reveal uses images throughout the RevealView such as the new visualization:

We exposed a way to change these images using the RevealView.assets. For example:

revealView.assets.dashboardEmptyState = new RevealApi.RVImageAsset( new RevealApi.RVImage("/images/dashboard_empty.png", "Empty Dashboard State Image"), "Add your First Visualization", "Visualize all your data in perfect harmony");