* 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