Merge pull request #12857 from dmatveev:hld

* G-API Documentation: first submission

This PR introduces a number of new OpenCV documentation chapters for
Graph API module.

In particular, the following topics are covered:
- Introduction & background information;
- High-level design overview;
- Kernel API;
- Pipeline example.

All changes are done in Markdown files, no headers, etc modified.

Doxygen references for main API classes will be added later.

Also, a tutorial will be introduced soon (in the common Tutorials place)

* G-API Documentation - fix warnings & trailing whitespaces

* G-API Documentation: address review issues

* G-API Documentation: export code snippets to compileable files

* gapi: move documentation samples

modules/gapi/include/opencv2/gapi/cpu/gcpukernel.hpp

@@ -8,10 +8,10 @@
 #ifndef OPENCV_GAPI_GCPUKERNEL_HPP
 #define OPENCV_GAPI_GCPUKERNEL_HPP
 
-#include <vector>
 #include <functional>
-#include <map>
 #include <unordered_map>
+#include <utility>
+#include <vector>
 
 #include <opencv2/core/mat.hpp>
 #include <opencv2/gapi/gcommon.hpp>

modules/gapi/include/opencv2/gapi/gkernel.hpp

@@ -10,11 +10,11 @@
 
 #include <functional>
 #include <iostream>
-#include <unordered_map> // map (for GKernelPackage)
-#include <vector>  // lookup order
 #include <string>  // string
-#include <utility> // tuple
 #include <type_traits> // false_type, true_type
+#include <unordered_map> // map (for GKernelPackage)
+#include <utility> // tuple
+#include <vector>  // lookup order
 
 #include <opencv2/gapi/gcommon.hpp> // CompileArgTag
 #include <opencv2/gapi/util/util.hpp> // Seq