migrate recoding benchmark to PLNK (dragoon/komodo!89)

this MR
- moves the last "recoding" benchmark to `examples/benches/`
- moves the README, which is now all alone, to `examples/benches/`
- adds a mention to `examples/benches/README.md` in `README.md`
- some minor improvements to the bench README

## TODO
- [x] find a way to plot the "recoding" results (thanks to !90)