Implement a custom CI job for auto Django model diagram generation

As a dev, I would like to have a custom CI job 
that automatically generates, updates, and verifies Django model diagrams, 
so that we can ensure consistent and up-to-date documentation across all Django-based repositories 
without manual intervention.

Background

In our current setup, each Django project (such as card-api, photo-api, and orgchart) has its own script (generate-data-model-diagrams.sh) to generate Django model diagrams. This process is manual and prone to inconsistencies across projects. By creating a standardized CI job, we can automate this process, reduce redundancy.

Acceptance Criteria

• The CI job should be defined in a shared CI template that can be included in the .gitlab-ci.yml files of Django-based projects.
• The job must automatically generate a Django model diagram when a merge request is created or updated.
• The generated diagrams should be stored in a designated directory (e.g., docs/data-model/) within the project repository.
• Documentation must be updated.