style(docs): add subtle shadow to screenshots

[Antreesy]

☑️ Resolves

• Fix blending of light-theme screenshots with the background
• Not visible on dark-theme docs, but it's a step forward nevertheless

🖼️ Screenshots

Before	After

[jancborchardt]

Looks good! :) Don’t we use variables here as well tho?
It’s fine to also use some bigger border-radius for images here, could be our border-radius-large: 10px;

[Antreesy]

Don’t we use variables here as well?

I see only hardcoded values, so guess not. Adjusted, added a dark-theme shadow as well