style: update background colors of codeblocks

[jsparkdev]

Overview

• update the background color of codeblocks to improve readability.
• add missing inline code.

codeblock background

before

after

Checklist

• Did you write the test code?
• Have you run yarn test:coverage to make sure there is no uncovered line?
• Did you write the JSDoc?

[Copilot]

Pull Request Overview

This PR updates the contributing documentation in both Korean and English to improve the readability of code blocks and to include missing inline code.

• Updated the background color in code block examples from white to #161618.
• Adjusted inline code formatting for branch references.

Reviewed Changes

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

File	Description
src/docs/ko/contributing.md	Updated code block background color to improve readability.
src/docs/en/contributing.md	Updated code block background color and formatted the branch name inline.

[seungrodotlee]

I think this sould consider both light and dark theme.

[seungrodotlee]

Thanks! 👍