doc: update concepts overview diagram

[raphaelauv]

closes : https://github.com/apache/airflow/issues/21071

[kaxil]

Please use the diagram over here: https://cwiki.apache.org/confluence/display/AIRFLOW/Drawio+Diagrams so that we can upload it over there again using Drawio or Lucidchart

[github-actions[bot]]

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