DOC: adds noise covariance to glossary (#6875)
DOC: adds noise covariance to glossary (#6875)

* adds noise covariance to glossary

* addressed comments for noise covariance glossary entry

* added cross ref to noise covariance

* convert tabs to spaces

rkmaddox/mne-python,cjayb/mne-python,Eric89GXL/mne-python,kingjr/mne-python,kingjr/mne-python,drammock/mne-python,pravsripad/mne-python,olafhauk/mne-python,kambysese/mne-python,larsoner/mne-python,olafhauk/mne-python,larsoner/mne-python,Teekuningas/mne-python,larsoner/mne-python,Teekuningas/mne-python,cjayb/mne-python,bloyl/mne-python,wmvanvliet/mne-python,Teekuningas/mne-python,olafhauk/mne-python,rkmaddox/mne-python,mne-tools/mne-python,pravsripad/mne-python,pravsripad/mne-python,drammock/mne-python,wmvanvliet/mne-python,wmvanvliet/mne-python,drammock/mne-python,Eric89GXL/mne-python,kambysese/mne-python,kingjr/mne-python,bloyl/mne-python,mne-tools/mne-python,mne-tools/mne-python