Improve existing documentation for the stark-felt crate.

[nils-mathieu]

This pull request provide some additional documentation to the methods defined for the Felt type.

Important Notes

One of the examples I made for inverse_mod do not pass. Here is the bogus example:

use stark_felt::{Felt, NonZeroFelt};
let a = Felt::from(3);
let p = Felt::from(5).try_into().unwrap();

let b = a.inverse_mod(&p).unwrap();
assert_eq!(a.mul_mod(&b, &p), Felt::ONE);

This do not pass.

Apparently, b is computed to be 4 instead of 2.

I'll open an issue for that.

State

Blocked by #11 .