refactor(example): enhance example for public inputs validation 2

[uri-99]

Note

This PR comes from a rebase of the original branch solving this issue
Original PR is this one
This branch will be merged and not the other one, to avoid git history conflicts

Update the public input validation example to support both SP1 and Risc0 implementations. Additionally, improve the steps and documentation to make it easier to follow and execute.

How to test

You can test it locally following the commands in examples/validating-public-input/README.md or use holesky following the official documentation in docs/guides/3_validating_public_input.md.

[JulianVentura]

After deleting the file sp1_fibonacci.elf if I run the command make generate_sp1_fibonacci_proof the execution fails because it's missing that file. Same happens if I execute cargo build on sp1/fibonacci/script directory. Seems the build.rs is not being executed for some reason.

[IAvecilla]

@JulianVentura It should be already fixed, the build.rs should run everytime now without the need of cleaning or building all the dependencies again.

[Oppen]

Looks OK. Haven't tested it tho.

[JuArce]

I need to finish the review later

[MarcosNicolau]

Other than the minor comments, everything worked fine.