DOC: added small doc on sct_register_pipeline
DOC: added small doc on sct_register_pipeline

Former-commit-id: 5b05d4f84926a3af4f95950b1ce993068d5a520f
Former-commit-id: 0001d8de210e41aafb899fcc0c292cab460c8f0e
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox