set up resampling cheatsheet

[lisa-wm]

idea: build look-up with most important properties of resampling strategies at a glance (appropriate with few data, bias/var, ...)
kxp CV, pxk CV, k CV, holdout, BS, subsampling, LOO, LOOO, stratification, ...

[ludwigbothmann]

Add some sentences to the cheatsheet on tuning

[berndbischl]

we have the sheet