Fixed examples

The examples were still using `core::` instead of `std::`
All of the examples needed a `use std::rand;` at the top to compile
Most of the examples had
    `rng = rand::rng();`
instead of
    `let mut rng = rand::rng();`
auto merge of #7693 : korenchkin/rust/fixdoc_rand, r=cmr

All of the examples were still using `core::` instead of `std::` and  needed a `use std::rand;` at the top to compile
Most of the examples had
    `rng = rand::rng();`
instead of
    `let mut rng = rand::rng();`