Add IMU explainers uniformly for Acq Board and ONIX Source plugins

[ChucklesOnGitHub]

The ONIX Source plugin includes the explanation of the data streaming from the IMU that the Acq Board doesn't. https://open-ephys.github.io/gui-docs/User-Manual/Plugins/Onix-Source.html#bno055-configuration

This would be stronger also linked to the IMU data wiki article.

Both source plugin pages would benefit from pointing to a commutator tutorial.
The ONIX Source page has an admonition that the Acq Board docs does not have:

Even though a commutator tutorial for the Acq Board docs exists but there isn't one for ONIX.
(Which is also sth we need to discuss adding open-ephys/commutator-docs#42 (comment))