Style `qiskit-card` directive with Furo

[Eric-Arellano]

Part of https://github.com/Qiskit/qiskit_sphinx_theme/issues/327.

We use the same general design as Pytorch, but with several improvements:

• The image stays a fixed size so that the aspect ratio is respected better and it doesn't constantly shrink and expand.
• Uses flexbox rather than media queries. This results in a much smoother experience when resizing the screen.
• Gets rid of a weird black overlay we were putting over the image. Instead, now we set opacity to 0.7 when not highlighted.
• Adds a border so that there is a more distinct flat design.

Pytorch design: