[DefApp] Document how it works

[miniksa]

@miniksa and @cinnamon-msft to document how this works.

_Originally posted by @miniksa in #7489

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1. Document how the app extension goo works
2. document the com server interface
3. probably add a way to do defterm unpackaged as well

[driver1998]

Implements the default application behavior and handoff mechanisms between console and terminal. The inbox portion is done already.

Could you hint us about what Windows version will support this feature?