Restructure doc as overview, tutorials, references

doc/source/brain.rst

doc/source/comparing.rst

doc/source/example/moving/multi_agent.py

doc/source/index.rst

doc/source/moving.rst

doc/source/overview/brain.rst

doc/source/overview/comparing.rst

doc/source/overview/database.rst

doc/source/overview/examples/Cam_blender.svg

doc/source/overview/examples/Python_blender.svg

doc/source/overview/examples/apcv.py

doc/source/overview/examples/asv_brain.py

doc/source/overview/examples/blenddemo_beesampling.py

doc/source/overview/examples/blenddemo_cyberbee.py

doc/source/overview/examples/contrast_weighted_nearness.py

doc/source/overview/examples/get_posorients.py

doc/source/overview/examples/load_image_posorient.py

doc/source/overview/examples/load_image_rowid.py

doc/source/overview/examples/michelson_contrast.py

doc/source/overview/examples/pcv.py