GitLab CI Documentation for v4.0.0 Release - Environment Variable Changes for Unity License Activation

The [GitLab CI Unity3D example project](https://gitlab.com/game-ci/unity3d-gitlab-ci-example/) has recently been updated with breaking changes [in version 4.0.0](https://gitlab.com/game-ci/unity3d-gitlab-ci-example/-/releases/v4.0.0). These changes affect how Unity licenses are activated and the required environment variables for the CI pipeline. The documentation on the [game.ci](https://game.ci) website needs to be updated to reflect these changes to ensure that users can continue to activate Unity licenses without issues.

Related website page to update:
- https://game.ci/docs/gitlab/activation
We should get closer to the following documentation:
- https://game.ci/docs/github/activation/

### Changes to Be Updated in the Documentation:

1. **Unity License Activation Update**:
- The `UNITY_USERNAME` variable has been replaced with `UNITY_EMAIL`.
- The `UNITY_SERIAL` environment variable has been made **mandatory** for activation and must be extracted from the Unity license file (`.ulf`). _This might be a temporary change until we have a better solution_
- **Breaking change**: The old `get_activation_file.sh` script has been removed. Users should now use the updated activation process, which involves Unity's blank project for license verification.
- Documentation should reflect these new changes and include updated instructions for retrieving and setting up the necessary environment variables (`UNITY_EMAIL`, `UNITY_PASSWORD`, `UNITY_SERIAL`).

2. **CI/CD Configuration Changes**:
- The `.gitlab-ci.yml` example configuration should be updated to reflect the new environment variables and the updated license activation process.
- Users of earlier versions should be instructed to update the `ci` folder containing the different shell script for activation and returning licenses.
3. **Docker Image Updates**:
- Ensure the documentation reflects the fact that the project now supports v3 Docker images.
- Provide guidance on how to adjust existing CI configurations to prevent errors caused by using outdated images.

4. **New Instructions for Unity License File**:
- Detailed steps on how to extract the `UNITY_SERIAL` from the `.ulf` file should be added.
- Update instructions for creating and setting the new environment variables (`UNITY_EMAIL`, `UNITY_PASSWORD`, `UNITY_SERIAL`) in GitLab's CI/CD settings.

### Suggested Documentation Updates:

1. **Activation Section**:
- Update instructions to specify that `UNITY_EMAIL`, `UNITY_PASSWORD`, and `UNITY_SERIAL` are now required as environment variables.
- Provide updated steps for retrieving and configuring these variables.
- Update the section where users configure their Unity credentials in GitLab CI/CD settings to reflect the new environment variable names.

2. **License Handling**:
- Clarify that Unity’s new activation mechanism requires the serial to be extracted from the `.ulf` file using a new script and add these steps to the documentation.