Fixes#1895
Changed most `println` to `info` in examples, some to `warn` when it was useful to differentiate from other more noisy logs.
Added doc on `LogPlugin`, how to configure it, and why (and how) you may need to disable it
* move print diagnostics to log
* entity count diagnostic
* asset count diagnostic
* remove useless `pub`s
* use `BTreeMap` instead of `HashMap`
* get entity count from world
* keep ordered list of diagnostics