Add env overloads for inplace DeviceScan (#8474)

* Add env overloads for inplace Scan

* Document and constrain inplace DeviceScan env overloads

* Add literalinclude blocks to all 5 new in-place env overloads
* Add missing ExclusiveScan(FutureValue) in-place tests
* Constrain new in-place env overloads with !is_integral_v<EnvT> to prevent EnvT from silently deducing to num_items when the user calls the non-inplace overload with env defaulted.
* Constrain existing non-inplace env overloads with is_integral_v<NumItemsT> to cut the symmetric ambiguity where NumItemsT would deduce to an env object.
* Align docs throught env and non-env overloads
* Wrap the EnvT default in the _CCCL_DOXYGEN_INVOKED guard to match the non-inplace style so Doxygen can render the signatures.

* Make enable_if actual template parameter

Author: gonidelis