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/samples/api_example.cpp

@@ -0,0 +1,34 @@
+#include <opencv2/videoio.hpp>
+#include <opencv2/highgui.hpp>
+#include <opencv2/gapi.hpp>
+#include <opencv2/gapi/core.hpp>
+#include <opencv2/gapi/imgproc.hpp>
+
+int main(int argc, char *argv[])
+{
+    cv::VideoCapture cap;
+    if (argc > 1) cap.open(argv[1]);
+    else cap.open(0);
+    CV_Assert(cap.isOpened());
+
+    cv::GMat in;
+    cv::GMat vga      = cv::gapi::resize(in, cv::Size(), 0.5, 0.5);
+    cv::GMat gray     = cv::gapi::BGR2Gray(vga);
+    cv::GMat blurred  = cv::gapi::blur(gray, cv::Size(5,5));
+    cv::GMat edges    = cv::gapi::Canny(blurred, 32, 128, 3);
+    cv::GMat b,g,r;
+    std::tie(b,g,r)   = cv::gapi::split3(vga);
+    cv::GMat out      = cv::gapi::merge3(b, g | edges, r);
+    cv::GComputation ac(in, out);
+
+    cv::Mat input_frame;
+    cv::Mat output_frame;
+    CV_Assert(cap.read(input_frame));
+    do
+    {
+        ac.apply(input_frame, output_frame);
+        cv::imshow("output", output_frame);
+    } while (cap.read(input_frame) && cv::waitKey(30) < 0);
+
+    return 0;
+}