feat: add JsonPlaceholder test collection with public API tests

- Add apiTest/JsonPlaceholder/opencollection.yml
- Add Users/Get all users.yml (GET /users test)
- Add Posts/Get post by ID.yml (GET /posts/1 test)
- Add Posts/Create new post.yml (POST /posts test)
- Update README.md with JsonPlaceholder reference

Tests use public JSONPlaceholder API - no local backend required.
Works in CI/CD pipeline automatically.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

README.md

@@ -182,7 +182,7 @@ Soubor: [`.github/workflows/api-tests.yml`](./.github/workflows/api-tests.yml)
 
 ---
 
-## 📞 Kontakt & Podpora
+## 📞 Podpora
 
 - **Issues**: [GitHub Issues](https://github.com/kankys/api_test/issues)
 - **Diskuse**: Použijte GitHub Discussions