docs: Add missing form section to Japanese principles documentation

[wo-o29]

📝 Key Changes

Added a missing section about wrapping input elements in <form> tags to the Japanese principles documentation, aligning it with the Korean original documentation structure.

Problem

• The Japanese principles page was missing a link to the form documentation under section 3 (Predictable Interactions)
• This created inconsistency between the Korean and Japanese documentation

Solution

• Added the missing link: [入力要素は<form>で包む](./predictability/form.md) under section 3
• This brings the Japanese documentation in line with the Korean version which includes this section
• Ensures complete coverage of the 4 core accessibility principles in Japanese

🖼️ Before and After Comparison

Before	After

[vercel]

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