* First draft of nGraph documentation
* updated according to review comments
* Updated
* Reviewed the nGraph Transformation section, added missing images
* Update nGraph_dg.md
* Delete python_api.md
Removed since there is already the nGraph_Python_API.md document with a comprehensive overview.
Co-authored-by: Andrey Zaytsev <andrey.zaytsev@intel.com>
Co-authored-by: CCR\avladimi <anastasiya.ageeva@intel.com>
