py-nutshell: Add how to enumerate in iteration
py-nutshell: Add how to enumerate in iteration

Resolves #3924.

Modified from suggestion in issue to include `kv()`’s other use case in maps,
matching Python’s `dict.items()`.
MasterDuke17/doc,tbrowder/doc,MasterDuke17/doc,MasterDuke17/doc,tbrowder/doc,MasterDuke17/doc,tbrowder/doc,tbrowder/doc