Documentation on loading an image from `src/main/webapp`.

[mvysny]

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new Image("/images/foo.png") does not work when the WAR is deployed in a non-empty context root.

Also, there's a zero docs on Image itself.

[mvysny]

The only documentation I could find is this excellent stack overflow post: https://stackoverflow.com/questions/57553973/where-should-i-place-my-vaadin-10-static-files/57553974#57553974