doc: filters: Rewrite the example image generator

The example image generator is now generic, yay!

* generate median images automatically

* update the parent asciidoc docs accordingly

* update python filters docs

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
doc: filters: Further example generator changes

* Apply filter function on small image directly
  (do not use resized big image)

* Add edge_sharpening filter

* change filters and python filters docs accordingly

Signed-off-by: Cyril Hrubis <metan@ucw.cz>