Change Docker Hub references to Docker Store

[smbkr]

File: engine/getstarted/step_three.md

Should be updated to reflect that the docker/whalesay image is now located at https://store.docker.com/community/images/docker/whalesay and not at Docker Hub. It would probably be beneficial to explain the difference between Docker Store and Hub.

[mdlinville]

This should be throughout the doc set.

[shubheksha]

I'll take this one up!

[shubheksha]

Do all links referring to images in the hub be changed to Docker Store?

[johndmulhausen]

This topic no longer exists.