Merge pull request opencv#18387 from dmatveev:dm/slides_upd_44

Update G-API slides to OpenCV 4.4

* G-API: Updated slides to v4.4 (+ sample)

* Slight formatting changes + Python API page

* Some more updates to slides:

- Added more info on 4.2 and 4.4 versions
- Added explanation on Operations and their functional wrappers