docs: sphinx: add a README to document usage and meta
docs: sphinx: add a README to document usage and meta

Former-commit-id: bb2c97fcc70a1f19e47b0f166a3bcab6eb46ba1d
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox