Establish/Clarify Ownership and Responsibility for Code Maintenance

Problem

We need to define expectations for long-term maintenance of modules or Python packages within the codebase.

Definition of done

Codebase Ownership/Maintenance guide is merged into https://docs.gitlab.com/ee/development/python_guide/ ### Maintainer responsibilities

Requirements

• Define a list what Maintainers should know as Python engineers
• Define a list of their responsibilities beside reviewing code
• Define a list of their goals while reviewing code