[AIRFLOW-XXXX] Add architecture section to k8sexec docs

[dimberman]

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Issue link: Document only change, no JIRA issue

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[mik-laj]

Can you add some labels to the diagram, please? The diagram uses the Airflow logo to describe many different components. It is not clear what the role of each logo is. Lines appear in the diagram, but they are not described. I think that it is also worth numbering the next steps in the diagram, which allows for easier tracking of the message. If possible, it is worth using generally accepted conventions, e.g. sequence diagram. This makes the drawings simpler to understand and easier to draw. You could use online editor e.g. https://www.planttext.com/
Example: https://github.com/apache/airflow/pull/6961/files
Old preview: https://user-images.githubusercontent.com/12058428/71550253-ebd62000-29cb-11ea-827f-d6d66cf25256.png
What do you think about Did you have any reason to draw this diagram in a graphics program? I am afraid of such diagram, because then they are difficult to edit.

I have one more comment. Can you maintain uniform naming between text and diagram also, please? You use the term "the backend DB" in the text, but it does not appear in the image, but instead is "PostgresSQL".

I also prepared preview of your documentation:
Preview: https://acceptable-goat.surge.sh/executor/kubernetes.html