* 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

* rewrote the line segment intersection function to make the static analyzer happy

* fixed bug with improper "no intersection" detection in some of corner cases

* fixed bug with improper "no intersection" detection in some of corner cases

* Document distance functors in dist.h

* Add spec for Distance

* Generate appropriate links for symbols

* G-API Fluid basic heterogeneity support: initial upload

* G-API Fluid heterogeneity: address some coding style issues

* G-API Fluid heterogeneity: fix compiler warnings

* G-API Fluid heterogeneity: fix warnings on Windows & ARMv7

* G-API Fluid heterogeneity: finally fix Windows warnings

* G-API Fluid heterogeneity: fix dangling reference problem

Relevant guards can be found in
https://github.com/opencv/opencv/blob/ef5579dc8667e5eb5e149acc4af898421eed99da/modules/features2d/src/kaze/AKAZEConfig.h#L8
and
https://github.com/opencv/opencv/blob/ef5579dc8667e5eb5e149acc4af898421eed99da/modules/ml/include/opencv2/ml.hpp#L44

Exceptions caught by value incur needless cost in C++, most of them can
be caught by const-reference, especially as nearly none are actually
used. This could allow compiler generate a slightly more efficient code.

- This is to accommodate the variabiilty in floating-point operations in new platforms/compilers
- Specifically due to the error margin found in NVIDIA Jetson TX2