Convert svg to png images in cross validation explanations

Firefox has issues to render the svgs.

changed milestone to %Workshop 3rd edition (2020-2021)

created merge request !31 (merged) to address this issue

mentioned in merge request !31 (merged)

mentioned in commit e2719933

closed with merge request !31 (merged)

closed with commit e2719933

mentioned in commit f3816b07

reopened

created merge request !33 (merged) to address this issue

mentioned in merge request !33 (merged)

closed with merge request !33 (merged)

mentioned in commit 44a81e93