update the data2bids examples for the BIDS motion

[robertoostenveld]

As discussed with @sjeung and @JuliusWelzel, I will rerun the data2bids and update the https://www.fieldtriptoolbox.org/example/bids_motion/ page (and the data on the download server) with the recent changes in fieldtrip/fieldtrip#2036.

Furthermore, I will extend the page with the XDF example from Sein.

[robertoostenveld]

The PR #2034 with updates to data2bids has now been merged.

I have already downloaded and rerun the existing motion examples, they still work fine. I still need to update the example page.

[robertoostenveld]

I have just added a short description of processing and converting XDF data to bids in this commit.

@sjeung, feel free to elaborate on your example that is included. Some background info would be useful (how did the experiment look like), some information on the processing of the Euler angles, etc. I suggest to keep that all at the start of the "example" section and separate from the code.