docs: use Stylelint setup wizard and stylelint.config.mjs - #53

What was wrong

code quality/03-stylelint.md was out of date with the current Stylelint get-started guide:

• It said "There is no setup wizard like ESLint has". Stylelint now has one: npm create stylelint@latest.
• It told students to create .stylelintrc.json themselves. The wizard and the docs now use stylelint.config.mjs.

What changed

• The setup command is now npm create stylelint@latest, with a short description of what it does and the config file it writes.
• One sentence says .stylelintrc.json still works, for students who meet it in older projects.
• Typo fix: "build in VS Code CSS linting" is now "built-in".

The Run and VS Code sections are otherwise unchanged.

Verified

I tested this in a fresh npm create vite@latest vanilla project on Node 24.18.0 and npm 11.16.0.

• npm first asks Ok to proceed? (y) to install create-stylelint@1.0.1. The wizard then shows a summary of what it will do and asks one question, Continue?, which defaults to Yes.
• It wrote stylelint.config.mjs with exactly the content shown on the page, and installed stylelint 17.16.0 and stylelint-config-standard 40.0.0.
• npx stylelint "**/*.css" linted src/ and skipped node_modules.
• With stylelint.config.mjs replaced by an equivalent .stylelintrc.json, Stylelint reported the same problems.
• markdownlint reports 0 issues, and npm run build passes.

🤖 Generated with Claude Code