Gateway documentation - New topic - updating existing applications to use new gateway SDKs

[denyeart]

Provide documentation for each language-specific SDK on how to update from existing SDKs to the new gateway SDKs.

Perhaps provide a conceptual overview in ReadTheDocs, and then link to specific guidance in the Node, Java, and Go SDKs.

[joshhus]

Part of overhaul on Developing apps docs perhaps.