* implement TransferTo{Host,Device}Mapper

* do not support transfer to same type

* use {to,from}_numpy

* lint fixes

* add comment

* ruff

* disable spurious pylint warning

* add to docs

* more doc fixes

* datawrapper doc

* more doc fixes

* Improve mapper names

* Bring back numpy actx constructor docs

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Co-authored-by: Andreas Kloeckner <inform@tiker.net>