[docs] Document Determinism Properties

[felixhandte]

Is your feature request related to a problem? Please describe. I've just been asked for the nth time whether zstd is deterministic and launched into the same spiel about "yes, but in a constrained way, and why are you asking??". This is a recurring siren call that our users want, and we should write up a description of the properties Zstd does and doesn't provide / an FAQ / guidance.

Describe the solution you'd like We should write a doc we can link people to.

Describe alternatives you've considered We could not do it.