VIZ/DOC/ENH: add MNE rest example + smaller fixes

[dengemann]

This example is analogous to our latest work in MNE-Python [https://github.com/mne-tools/mne-python/pull/5692] and serves as "cross" validation of our code.

cc @larsoner @bloyl @ktavabi @agramfort

Here some outputs:

[larsoner]

Excellent!