CSS のボックスモデルの演習問題を削除

[nakaterm]

#891 により、「CSS によるレイアウト」の章でボックスモデル周辺の知識を説明するようになるため、「CSS」の章からはボックスモデルに触れる演習問題を削除

[Copilot]

Pull request overview

This PR removes the CSS box model exercise (Exercise 3) from the CSS chapter, as the box model concepts will be covered in a new "CSS Layout" chapter (per issue #891).

Key changes:

• Removed Exercise 3 (発展) which covered box model properties like margin, padding, border, border-radius, and box-shadow
• Deleted associated sample code and image resources

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
docs/1-trial-session/03-css/index.mdx	Removed Exercise 3 and its answer section covering box model styling
docs/1-trial-session/03-css/_samples/foo/index.html	Deleted sample HTML file demonstrating box model properties
docs/1-trial-session/03-css/rounded-box-with-shadow.png	Deleted screenshot image used in the removed exercise

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The changes in this PR are clean and complete. All references to the box model exercise have been properly removed, including:

• The exercise description and answer
• The sample code directory (_samples/foo)
• The associated image file
• The ViewSource component reference

No orphaned references remain, and the chapter now ends cleanly after Exercise 2. The removal aligns well with the stated goal of moving box model content to a dedicated layout chapter.

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