Improve the distinction between images and text

In most documentation pages, the screenshots have a white background, which makes them uneasy to distinguish from the text.

Add a CSS rule for all screenshots in order to add a margin on the left, and a thin black border around. There is currently no custom CSS file, so create one in "assets" directory.

For all figures with width="100%", reduce them to "90%", so that they don't overflow on the right (and make a horizontal scrollbar appear), and we have the same margin (5%) on the left and right sides.