docs: some images less accessible for dark GitHub themes

[ginglis13]

if using one of the GitHub dark themes, it is near impossible to view documentation images such as

https://github.com/firecracker-microvm/firecracker-containerd/blob/main/docs/img/design-overview.png

and

https://github.com/firecracker-microvm/firecracker-containerd/blob/main/docs/img/design-sequence-diagram.png

due to their usage of black text and transparent background.

It would be nice if we could convert these images to have a white/higher contrast background so they can be viewed using any GitHub theme.

[jxie0615]

Hi! Is this issue still open to be worked on? My partner and I are students from UT Austin, and we were wondering if we could tackle this as a good first issue for our virtualization class?