docs: add example how to print constraints in compiled constraint system

[ivokub]

Discussed in #1637

^{Originally posted by sshravan October 31, 2025}
Hi! Is there a way to view the compiled constraint system in gnark?

I am looking for something similar to the visual view available on https://play.gnark.io/, as mentioned in this PR discussion.

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This has been asked several times. I think minimally we should have an example in the examples directory. Or we could have a shorthand shortcut which does the type assertion.