Refactor: Remove obsolete fixture files and custom steps

[Grazulex]

• Deleted CustomSteps.php and related flow YAML files to streamline the test suite.
• Introduced GroupsFixture.php to manage step groups and nested flows.
• Added advanced group handling with TestGroupStep for better testing of flow functionalities.
• Updated documentation to reflect changes in flow processing examples and usage.

Description

Related Issue

Closes #

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Performance improvement
• Test improvement

Changes Made

Testing

• Tests added/updated for the changes
• All existing tests pass
• New tests pass
• Manual testing performed

Code Quality

• Code follows the project's style guidelines
• Self-review of code performed
• Code is properly documented
• No unnecessary debug code or comments left

Screenshots (if applicable)

Additional Notes

Checklist

• I have read the CONTRIBUTING.md guide
• My code follows the project's coding standards
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes