Resampling (chunk 1/3)

[topepo]

I have the resampling chapter fleshed out but decided to break it in to three parts:

1. Introduction; discussion of general strategies; validation sets; MC-CV; simulation
2. V-fold CV; bootstrapping; niche resampling methods
3. Resamping FAQ

Also - small edits to previous chapters where "data" were not treated as plural 🙀.

If you want the full firehose, I can delete this PR and give it to you all at once.