Add section on implementing the rand_core traits

[eldruin]

Since we will remove the RNG traits in embedded-hal itself, it would be good to have a section here about implementing the rand_core traits for a HAL where we can lead users to.

[jannic]

I looked through existing documentation for mentions of random number generation:

• https://docs.rust-embedded.org/discovery-mb2/16-snake-game/game-logic.html uses random numbers but directly access the rng of the underlying pac without using a higher level interface lite rand_core
• https://docs.espressif.com/projects/rust/book/introduction/ancillary-crates.html#embedded-rust-ecosystem-integration mentions that esp-hal implements rand_core. This is just an example of an implementation and no explanation.

I did not find any documentation on how to implement rand_core traits for embedded systems. So this ticket is not yet resolved.