update mftf section in readme

[piotrkwiecinski]

selenium configuration sits in compose.yaml instead of compose.dev.yaml

Closes #867

[what-the-diff]

PR Summary

• Rename compose.dev.yaml file to compose.yaml
  This update simplifies the file handling for future development.
• Update references in bin/mftf script
  All references are updated to match the new file name.
• Add SELENIUM_HOST line in the .env file
  This improves the Selenium integration with Magento and ensures tests run smoothly.
• Enable access to the dev section in Nginx config
  This allows for successful execution of MFTF tests without permission-related issues.