content: add new cultural etiquette tip

[gabriellim]

📝 Description

[...](content: add new cultural etiquette tip)

🔗 Related Issue

Closes #14643

🎯 Type of Change

• fix: Bug fix (non-breaking change which fixes an issue)
• feat: New feature (non-breaking change which adds functionality)
• docs: Documentation update (e.g., this file, README)
• content: Content update (e.g., new kanji, vocab, or fonts in /static/)
• style: UI/Theme changes (e.g., Tailwind, CSS, new themes)
• refactor: Code refactor (no functional changes)
• test: Test update (adding missing tests or correcting existing tests)
• chore: Build, CI/CD, or dependency updates

✅ Pre-Submission Checklist

• My code follows the project's code style and uses cn() utility where needed
• I have run npm run check locally and there are no TypeScript/ESLint errors 🐞
• I have starred the repo ⭐
• My commit messages follow the Conventional Commits format
• I have updated the documentation (if applicable) 📖
• This PR is against the main branch

🧪 How Has This Been Tested?

Test Steps:

1. ...
2. ...

📸 Screenshots/Videos (if applicable)

...

Helpful links: Contributing · Troubleshooting

📦 Additional Context

...

[tentoumushii]

🎉 Thanks for your Pull Request, @gabriellim!

We appreciate your contribution to KanaDojo!

Pre-merge checklist:

• You starred our repo ⭐
• Code follows project style guidelines
• Changes have been tested locally
• PR title is descriptive
• If this closes an issue, it's linked with Closes #<number>

A maintainer will review your PR shortly. In the meantime, make sure all CI checks pass. You can run npm run check locally to match CI.

ありがとうございます! 🙏

[tentoumushii]

🤖 Auto-Review: ✅ Passed

This cultural etiquette contribution has passed automated validation!

Checks passed:

• ✅ Changed files are in the allowed location
• ✅ Changed JSON files parse successfully
• ✅ Related issue found

📎 Linked issue: #14643

Once the quality check passes, this PR will be automatically approved for merge.