DOC: add "working with sensor locations" tutorial (#6394)
DOC: add "working with sensor locations" tutorial (#6394)

Adds a tutorial describing how to plot sensor locations, and how the physical
location of sensors is handled in MNE-Python.
kambysese/mne-python,kambysese/mne-python,drammock/mne-python,wmvanvliet/mne-python,larsoner/mne-python,Teekuningas/mne-python,pravsripad/mne-python,rkmaddox/mne-python,drammock/mne-python,mne-tools/mne-python,Teekuningas/mne-python,pravsripad/mne-python,Eric89GXL/mne-python,olafhauk/mne-python,bloyl/mne-python,wmvanvliet/mne-python,wmvanvliet/mne-python,bloyl/mne-python,Eric89GXL/mne-python,mne-tools/mne-python,rkmaddox/mne-python,kingjr/mne-python,kingjr/mne-python,Teekuningas/mne-python,cjayb/mne-python,mne-tools/mne-python,kingjr/mne-python,olafhauk/mne-python,drammock/mne-python,cjayb/mne-python,olafhauk/mne-python,larsoner/mne-python,pravsripad/mne-python,larsoner/mne-python