* Split api to onnx_utils, headers refactor

* Improved ONNX importer API doc

* styles applied

* Code review remarks introduced

* remarks introduced, style applied

* Code review remarks introduced

* namespaces description added

* Code review remarks introduced

* styles applied

* Fixed onnx_utils doc and margin alignment

* code review remarks introduced
Co-authored-by: Michał Karzyński <postrational@users.noreply.github.com>