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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[github-actions]

Size changes

📦 Next.js Bundle Analysis for react-dev

This analysis was generated by the Next.js Bundle Analysis action. 🤖

This PR introduced no changes to the JavaScript bundle! 🙌

[hirotomoyamada]

I have signed the Contributor License Agreement.

[chogyejin]

Isn't this PR a duplicate of #6927?

[hirotomoyamada]

@chogyejin

Oh, It seems so.