document how to create a local copy of a nimble demo app

can someone please document how to crate a local instance of a demo app (i.e. btshell, etc) so one might locally modify the code.

The docs show referencing the git repo `@apache-mynewt-nimble/apps/btshell` but that omits the ability to edit/build the target.

there must be a simple way to **fork** one of these examples and build/edit it locally.

Start with the documentation that references `@apache-mynewt-nimble/apps/btshell` and inspect how the btshell demo is currently obtained. Document the local workflow for creating an editable copy and building it; done means a newcomer can modify and build a demo app locally.

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