[KubeIngressProxy] Add a long docstring to KubeIngressProxy class

[consideRatio]

For a long time I've wanted to document the KubeIngressProxy class that ships with this project even though its not needed for KubeSpawner. In the past I've often been to confused to do so, but at this point I felt ready with a better comprehension of the class, its background, and role.

This isn't perfect, and relies on my memory of conversations. @yuvipanda can probably improve this documentation further if time permits.

I think it's an important improvement not only for KubeIngressProxy curious people, but especially for anyone contributing with maintenance of KubeSpawner itself that finds this code - they will need to understand the entire code base to know they are not making breaking changes.

Closes #163 for now I think.

[consideRatio]

Pinged @GeorgianaElena for review for your experience with JupyteHub Proxy classes and @yuvipanda for being the author of this Proxy class.

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PS: I'd like to have this merged after #563 resolves to avoid handling a merge conflict that I think could happen there otherwise. So, don't merge unless it has merged first.

[GeorgianaElena]

This is an awesome thing to do @consideRatio ❤️ You're great!

From what I understand, and a proxy perspective, the docstring looks very good and accurate! I've left some comments with suggestions that either add a few details, or just reword the phrases a bit, but this should be good to go!

Thank you!

[consideRatio]

@GeorgianaElena thank you so much for your swift, uplifting, and thorough review!!! Excellent suggestions you made!

[yuvipanda]

Thanks a lot for working on clarifying this, @consideRatio! I've left a couple of clarifications.

https://github.com/berkeley-dsep-infra/data8xhub was the entire config for the infrastructure I'd set up for supporting >20k active users under peak loads, and this was built for that - if you're interested in looking :)

[consideRatio]

@yuvipanda thanks for your input! I've rebased and added 04457e6 with the changes made since your review.

[yuvipanda]

Awesome, thanks a lot @consideRatio and @GeorgianaElena!