Document random library

[ss2165]

Guppy v1 adds a native PCG32 random number generator to the standard library, including bounded random integers and mutable encapsulated RNG state.

Relevant PRs:

• feat(std): add native PCG32 random number generator guppylang#1808
• feat(std): add bounded random int to PCG32 guppylang#1877
• feat!: Use mutable state for PCG32 and encapsulate internals guppylang#1828

Suggested scope:

• Document basic PCG32 initialization and sampling.
• Show bounded integer generation.
• Explain that RNG state is mutable and should be handled like other Guppy state.
• Explain when to use this and when to use the qsystem RNG

[CalMacCQ]

@hsemenenko Would you be interested in taking a stab at this? I'm happy to review/discusss structure of course. No pressure.