[DOCS] Legacy MO Extensibility update for master (#15931)

Divided MO Extensibility article into separate smaller articles,
Applied the suggestion from [DOCS] Better statement about MO extensions as internal API [Recreating #14062] #15679
Recreated images in svg format
Fixing directives

docs/Extensibility_UG/Intro.md

@@ -9,7 +9,6 @@
    openvino_docs_Extensibility_UG_add_openvino_ops
    openvino_docs_Extensibility_UG_Frontend_Extensions
    openvino_docs_Extensibility_UG_GPU
-   openvino_docs_MO_DG_prepare_model_customize_model_optimizer_Customize_Model_Optimizer
    
 .. toctree::
    :maxdepth: 1
@@ -18,6 +17,12 @@
    openvino_docs_transformations
    OpenVINO Plugin Developer Guide <openvino_docs_ie_plugin_dg_overview>
 
+.. toctree::
+   :maxdepth: 1
+   :hidden:
+   
+   openvino_docs_MO_DG_prepare_model_customize_model_optimizer_Customize_Model_Optimizer
+
 The Intel® Distribution of OpenVINO™ toolkit supports neural network models trained with various frameworks, including
 TensorFlow, PyTorch, ONNX, TensorFlow Lite, PaddlePaddle, Apache MXNet, Caffe, and Kaldi. The list of supported operations is different for
 each of the supported frameworks. To see the operations supported by your framework, refer to :doc:`Supported Framework Operations <openvino_docs_MO_DG_prepare_model_Supported_Frameworks_Layers>`.
@@ -54,7 +59,7 @@ Mapping of custom operation is implemented differently, depending on model forma
 
 1. If a model is represented in the ONNX (including models exported from Pytorch in ONNX), TensorFlow Lite, PaddlePaddle or TensorFlow formats, then one of the classes from :doc:`Frontend Extension API <openvino_docs_Extensibility_UG_Frontend_Extensions>` should be used. It consists of several classes available in C++ which can be used with the ``--extensions`` option in Model Optimizer or when a model is imported directly to OpenVINO runtime using the ``read_model`` method. Python API is also available for runtime model import.
 
-2. If a model is represented in the Caffe, Kaldi or MXNet formats, then :doc:`Model Optimizer Extensions <openvino_docs_MO_DG_prepare_model_customize_model_optimizer_Customize_Model_Optimizer>` should be used. This approach is available for model conversion in Model Optimizer only.
+2. If a model is represented in the Caffe, Kaldi or MXNet formats, then :doc:`[Legacy] Model Optimizer Extensions <openvino_docs_MO_DG_prepare_model_customize_model_optimizer_Customize_Model_Optimizer>` should be used. This approach is available for model conversion in Model Optimizer only.
 
 Existing of two approaches simultaneously is explained by two different types of frontends used for model conversion in OpenVINO: new frontends (ONNX, PaddlePaddle, TensorFlow Lite and TensorFlow) and legacy frontends (Caffe, Kaldi and Apache MXNet). Model Optimizer can use both front-ends in contrast to the direct import of model with ``read_model`` method which can use new frontends only. Follow one of the appropriate guides referenced above to implement mappings depending on framework frontend.