docs: streamline the contributing guide

What & Why

Rewrites CONTRIBUTING.md into a single, concise "Sending a pull request" flow (fork → install → branch + checks → commit + PR). Adds an explicit upstream remote step, documents the git-hook–enforced branch/commit conventions, and notes that there is no test suite so changes are verified manually. Also tidies the Contributing section formatting in README.md / README.ko.md to match.

Related Issue

N/A

How to Verify

1. Read CONTRIBUTING.md and follow the steps — repo URL, env vars, branch prefixes, and check commands all match the project.
2. Run npm run format:check — docs are Prettier-clean.
3. Open the Contributing section in README.md / README.ko.md and confirm it renders and links to CONTRIBUTING.md.

Checklist

• PR title follows Conventional Commits (feat:, fix:, chore:, refactor:, docs:, i18n:)
• Translations added to both locales/ko.json and locales/en.json (if UI text changed)
• Tested on mobile viewport (if UI changed)