* 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
