Image styling: borders

In a recent conversation about centering images, we also slightly touched on the contrast issue with screenshots originating from the product having the same background color as the docs page background. The fact that our images don't have borders impairs a bit the readability and scannability of our docs pages, especially ones where we need more screenshots than usual, such as visualization-related features.

This is a bigger issue now than in the past because of recent UI theme changes made to the product, where the white-ish background is a lot more present than in previous versions where the product background was grey more often, thus creating more natural contrast and distinctions on docs pages with screenshots.

Example:

Some options:
- auto-add a solid border to images
- make it optional using a new styling parameter (we could debate whether it's on or off by default)

Also a note that this shouldn't apply to inline images such as icons.

cc @theletterf