document / provide code on how to update anatomical landmarks after coregistration

[SophieHerbst]

We are processing a new dataset, for which we need to do the coregistration after the BIDS import, as we need to first recon-all our mris. If I am not mistaken, we need to run mne_bids.update_anat_landmarks() after obtaining the coregistration. I think this should be documented in the pipeline, or can we even provide a script to do it after the optional freesurfer steps?