Move selenium service & configuration to compose.dev.yaml

[piotrkwiecinski]

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

Closes #867

[what-the-diff]

PR Summary

• Updated File Reference in MFTF Setup Instructions
  The instructions now reference compose.yaml instead of compose.dev.yaml for better clarity when setting up the Flawed Model Turing Framework (MFTF).

• Corrected Step 2 Reference for Updating extra_host Values
  The second step now corrects refers to compose.yaml instead of compose.dev.yaml, when updating the extra_host values in the setup process. This makes it more user friendly and prevents confusion during setup.

[markshust]

I'm going to reopen this one as well -- the Selenium config should live in the compose.dev.yaml file.

[piotrkwiecinski]

This PR modifies documentation as currently selenium definition is located here https://github.com/markshust/docker-magento/blob/release/next/compose/compose.yaml#L138-L143

To my PR should be closed and code should be moved to compose.dev.yaml file instead.