docs: updated image URLs in markdown files

[hirotomoyamada]

Closes #6758 Closes #6924

Replace broken https://placekitten.com placeholder images with https://loremflickr.com.

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Name	Status	Preview	Comments	Updated (UTC)
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Size changes

## 📦 Next.js Bundle Analysis for react-dev This analysis was generated by the [Next.js Bundle Analysis action](https://github.com/hashicorp/nextjs-bundle-analysis). 🤖 This PR introduced no changes to the JavaScript bundle! 🙌

[hirotomoyamada]

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