Task: add data documentation for the new ETL pipeline

[nlebovits]

Describe the task

Add a proper data dictionary and a DAG for the new pipeline. This should include, at a minimum, the data sources, attributes, update frequency (per the API), and known issues.

Acceptance Criteria

• Data dictionary in markdown format added to the repo
• A DAG added to the repo, also ideally in some kind of reproducible/editable format (e.g., mermaid.js)