Cleanup `docs` directory

1) Remove FFT-related benchmarks and examples
2) Remove old code based on old-style IPython-parallel
3) Remove artifacts that are generated by the example scripts (`.png`s)
4) Add a README to the examples directory to remind us to run them with `mpiexec`

All the examples and benchmarks still present run (even after the `remove-extraneous-code` PR).