Comment RNG reproducibility wrapper on "Get started" page

[nmdefries]

In the "get started" vignette, this code and a variant with geo_value shows up here and here.:

withr::[with_rng_version](https://withr.r-lib.org/reference/with_rng_version.html)("3.0.0", withr::[with_seed](https://withr.r-lib.org/reference/with_seed.html)(42, [length](https://rdrr.io/r/base/length.html)(state)))

This is about making the RNG reproducible. We should add a comment.