Improve pre-commit to generate Airflow diagrams as a code

[potiuk]

Since we are getting more diagrams generated in Airflow using the "diagram as a code" approach, this PR improves the pre-commit to be more suitable to support generation of more of the images coming from different sources, placed in different directories and generated independently, so that the whole process is more distributed and easy for whoever creates diagrams to add their own diagram.

The changes implemented in this PR:

• the code to generate the diagrams is now next to the diagram they generate. It has the same name as the diagram, but it has the .py extension. This way it is immediately visible where is the source of each diagram (right next to each diagram)

• each of the .py diagram Python files is runnable on its own. This way you can easily regenerate the diagrams by running corresponding Python file or even automate it by running "save" action and generate the diagrams automatically by running the Python code every time the file is saved. That makes a very nice workflow on iterating on each diagram, independently from each othere

• the pre-commit script is given a set of folders which should be scanned and it finds and run the diagrams on pre-commmit. It also creates and verifies the md5sum hash of the source Python file separately for each diagram and only runs diagram generation when the source file changed vs. last time the hash was saved and committed. The hash sum is stored next to the image and sources with .md5sum extension

Also updated documentation in the CONTRIBUTING.rst explaining how to generate the diagrams and what is the mechanism of that generation.

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[potiuk]

This one should set us on a path where we should be able to convert pretty much all our architecture-like-diagrams to diagrams as a code approach easily.

It also allows to iterate on the diagrams very easily and explains how to integrate diagram "live preview" making it possible to instantly regenerate each diagram separately using "save actions".

[josh-fell]

This split is nice and more scalable indeed.

[vincbeck]

Feel free to merge without my approval (I am now away until next year)

[potiuk]

Feel free to merge without my approval (I am now away until next year)

Have good holidays :)