docs: add more "godoc-like" `ExampleXXX`

[gbotrel]

If they match the ones in docs.gnark.consensys.net, that would help too.

Things like "how to [de]serialize a proving key, circuit" or "how to test my circuit without running the prover" are recurrent questions.

[ivokub]

To add, I think there definitely has to be something at root package github.com/consensys/gnark. Right now when looking at package doc then it is really confusing where to start from. And there are so many sub-packages that it doesn't help (i.e. we describe writing circuits in frontend, but there is no direct reference to start from there).