EDU-2989: Add background to images to improve readability in dark mode

[tomwheeler]

What was changed

I added a white background to several images to provide contrast that will improve readability for users who have enabled dark mode.

Why?

As illustrated in EDU-2989, the images previously had transparent backgrounds. Dark lines and text in these images were difficult or impossible to read in dark mode, since that changes the page background from white to black. Since I don't use dark mode, I was unaware of the problem until a co-worker mentioned it.

Checklist

1. Closes

EDU-2989

2. How was this tested:

I verified that all images used in the documentation and previously had text or lines on a transparent background now have a white background to provide the required contrast regardless of whether a user has enabled dark mode.

4. Any docs updates needed?

No