Improve Cookbook page on randomness

[anton-trunov]

It's mostly a stub now: https://docs.tact-lang.org/cookbook/random/

We should better explain how to use our random, randomInt, nativePrepareRandom and nativeRandomizeLt functions, list common pitfalls, explain how validators can attack contracts using randomness and how those attacks can be mitigated.

[Gusarich]

how those attacks can be mitigated.

there's no way to protect from a theoretical validator attack on random-dependent smart contract. but in order to protect from regular users you just can call random/randInt (they call nativePrepareRandom by default) or nativeRandomizeLt once before all cals.