Describe the Variables section in documentation

[Oleksandr123234]

In the release of KubeRocketCI version 3.10.0, a new feature named "variables" was introduced. This feature allows users to define variables that can be used across their CI/CD pipelines, enhancing flexibility and reusability. To ensure our documentation accurately reflects the latest capabilities of our platform, it is necessary to update the documentation to include comprehensive information about this new feature.