Add Sphinx labels to all docs sub-pages
Rename: docs/tutorials/catalog/source_images.rst -> docs/tutorials/catalog/index.rst
Rename: docs/tutorials/npred/predicted_counts.rst -> docs/tutorials/npred/index.rst
Rename flux_point.rst -> flux_point/index.rst
Move flux_point tutorial files
Move crab_mwl_sed tutorial files
Move residuals_images.py to flux_point folder
Move docs/tutorials/image to docs/tutorials/gammapy-pfimage
Move docs/tutorials/spectrum to docs/tutorials/gammapy-pfspectrum
Update docs/tutorials index page
Move docs/tutorials/gammapy-pfspectrum/ docs/tutorials/gammapy-pfspec
Move docs/tutorials/gammapy-pfimage/ docs/tutorials/gammapy-pfmap