[docs] Add Checkbox and Number Field images

[danilo-leal]

• I have followed (at least) the PR section of the contributing guide.

Follow up to #288 (comment). Still iterating on the dark mode styles for these images. Not sure I'm fully happy with it yet 😬

[colmtuite]

Approved ✅ It's fine for now, but these should be done with CSS rather than <img /> for a few benefits:

1. We don't need to maintain an external file in Figma.
2. Improved perf since we don't need to download ~50 images.
3. We get dark mode for free. No need to draw, maintain, load separate images.
4. We can do micro animations if we want.

[danilo-leal]

Capturing the "convert to CSS" goal in issues so we don't lose out of sight:

• [material-ui][docs] Convert the "all components" images to CSS material-ui#41856
• [docs] Convert the "all components" images to CSS #293
• [charts][docs] Convert the "all components" images to CSS mui-x#12743