🐛 [BUG] - Update placeholder images in docs

[joshfarrant]

Describe the bug

The domain we were previously using for our placeholders — placeholder.com — has been sold and is no longer a placeholder image site. This has caused the images in our docs to 404.

We should find a replacement — remote or local — and update our images to use that replacement.

Note: we need to consider colour contrast when choosing a replacement placeholder image, see https://github.com/github/primer/issues/3737

Reproduction steps

1. Go to https://primer.style/brand/components/Card/react#image
2. Note the missing placeholder image

Expected behavior

1. Go to https://primer.style/brand/components/Card/react#image
2. A placeholder image is rendered

Screenshots

Browsers

No response

OS

No response