ngraph docs to master (#2568)

* 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.

* Fixed links to images

Co-authored-by: Andrey Zaytsev <andrey.zaytsev@intel.com>
Co-authored-by: CCR\avladimi <anastasiya.ageeva@intel.com>

docs/nGraph_DG/nGraph_basic_concepts.md

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+# nGraph Basic Concepts {#openvino_docs_nGraph_DG_basic_concepts}
+
+The nGraph represents neural networks in uniform format. User can create different operations and combined their to one `ngraph::Function`.
+
+## nGraph Function and Graph Representation <a name="ngraph_function"></a>
+
+nGraph function is a very simple thing: it stores shared pointers to `ngraph::op::Result` and `ngraph::op::Parameter` operations that are inputs and outputs of the graph. 
+All other operations hold each other via shared pointers: child operation holds its parent (hard link). If operation has no consumers and it's not Result operation
+(shared pointer counter is zero) then it will be destructed and won't be accessible anymore. Each operation in `ngraph::Function` has a `std::shared_ptr<ngraph::Node>` type.
+
+For details on how to build an nGraph Function, see the [Build nGraph Function](./build_function.md) page.
+
+## Operations
+
+`ngraph::Op` represents any abstract operations in the nGraph representation. You need to use this class to create [custom operations](../IE_DG/Extensibility_DG/AddingNGraphOps.md).
+
+## Operation Sets
+
+Operation set represents the set of some nGraph operations. `nGraph::Opset` is a class which provide a functionality to work with operation sets.
+Custom operation set should be created to support custom operation. Please read [Extensibility DevGuide](../IE_DG/Extensibility_DG/Intro.md) for more details.
+
+## Static and Partial Shapes
+
+nGraph has two types for shape representation: 
+
+* `ngraph::Shape` - Represents static (fully defined) shapes.
+
+* `ngraph::PartialShape` - Represents dynamic shapes. That means that the rank or some of dimensions are dynamic (undefined). `ngraph::PartialShape` can be converted to `ngraph::Shape` using the `get_shape()` method if all dimensions are static; otherwise the conversion raises an exception.
+
+For examples, see the Dynamic Shape and Rank section in the [Overview of Transformations API](./nGraphTransformation.md).
+
+
+## Transformation API
+
+nGraph transformation API allows you to manipulate the graph represented by `nGraph::Function`. For more details, see the [Overview of Transformations API](./nGraphTransformation.md) section.
+
+## Pattern Matcher
+
+For more details, see the Pattern Matching section in the [Overview of Transformations API](./nGraphTransformation.md).