docs: document how React handles HTML attributes in JSX

[suii2210]

This PR adds a new subsection "How React Handles HTML Attributes" to
src/content/learn/writing-markup-with-jsx.md.

It documents:

• JSX vs HTML attribute name differences (className, htmlFor)
• boolean attributes
• inline style objects
• data-* and aria-* attributes
• handling unknown/custom attributes and helpful tips

Fixes #80.

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[github-actions]

Size changes

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This PR introduced no changes to the JavaScript bundle! 🙌