Document options for exporting datasets with derived variables

[sfmig]

Is your feature request related to a problem? Please describe.
Currently we only document how to save movement poses datasets to common pose estimation formats (see here). In practice this means one can only export position data, but not velocity, acceleration, etc.

Describe the solution you'd like
Describe how we can use xarray's in-built methods to export movement datasets to common formats.

Maybe we can select ~2 of the methods documented in xarray that we think our users will be most familiar with and find most useful?

We then would mention them in the getting started guide, and maybe make an example for data exporting (to numpy, pandas or files, using xarray functionalities).

Describe alternatives you've considered
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Additional context
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