[DOC] Model caching feature overview (#5519)

* Docs: Model caching feature overview

* Update docs/IE_DG/Intro_to_Performance.md

Co-authored-by: Anastasiya Ageeva <anastasiya.ageeva@intel.com>

* Apply suggestions from code review

Co-authored-by: Anastasiya Ageeva <anastasiya.ageeva@intel.com>

* Review comments
- Moved code examples to snippets
- Added link to Model Caching overview from "Inference Engine Developer Guide"
- Few minor changes

* Update docs/IE_DG/Intro_to_Performance.md

Co-authored-by: Anastasiya Ageeva <anastasiya.ageeva@intel.com>

Co-authored-by: Anastasiya Ageeva <anastasiya.ageeva@intel.com>

docs/snippets/InferenceEngine_Caching1.cpp

@@ -0,0 +1,13 @@
+#include <ie_core.hpp>
+
+int main() {
+using namespace InferenceEngine;
+    std::string modelPath = "/tmp/myModel.xml";
+    std::string device = "GNA";
+    std::map<std::string, std::string> deviceConfig;
+//! [part1]
+    InferenceEngine::Core ie;                                 // Step 1: create Inference engine object
+    ie.LoadNetwork(modelPath, device, deviceConfig);          // Step 2: LoadNetwork by model file path
+//! [part1]
+return 0;
+}