Add docs about extending the Airflow helm chart

[shohamy7]

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This PR add docs about how users can extend the Airflow chart easily.
Writing this doc was suggested by @potiuk in the following (closed) PR: #36294
This should resolve the issue: #34399
Resolve: #34399
Instead of adding unnecessary and unrelated templates to our chart, we add a documentation about how the users could do that themselves.

[shohamy7]

@potiuk please review, would like to here what you think :)

[potiuk]

This is a very good start! I think we can add some examples later (or maybe let people contribute the examples) :)

Thanks for doing tit @shohamy7 !

[potiuk]

I am all for merging it as-is and then adding incrementally some examples later :)