add new page on sampling keyword arguments

[penelopeysm]

This should, hopefully, document a bunch of things that were not documented.

Closes #647.

[github-actions]

Preview the changes: https://turinglang.org/docs/pr-previews/654
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[joelkandiah]

It may make sense to specify exactly how thinning works in a similar way to the table in Thinning and warmup. This is because the thinning only applies to steps after discard_initial. This partiallye exists already, however doesn't note the interaction with num_warmup.

This is relevant as if I specify sample(..., N = 100, ...; discard_initial = 100, thinning = 10, num_warmup = 500 ) the code should call step_warmup 500 times and then step 591 times, which makes sense but may be unintuitive for first time users (i.e. N only starts counting from after discard initial, and N-1 multiplied by thinning and counts from 1 rather than 0, but num_warmup counts from the first discard iteration).

[joelkandiah]

Happy with this page now, it has covered all of the features I believe to be regularly used.

[penelopeysm]

Thanks Joel!