Restructure doc as overview, tutorials, references

doc/source/overview/examples/ridf.py

doc/source/overview/examples/scene.py

doc/source/overview/examples/skyline.py

doc/source/overview/examples/static_brain.py

doc/source/overview/index.rst

doc/source/overview/moving.rst

doc/source/overview/processing.rst

doc/source/overview/rendering.rst

doc/source/processing.rst

doc/source/references/brain.rst

doc/source/references/comparing.rst

doc/source/references/database.rst

doc/source/references/index.rst

doc/source/references/moving.rst

doc/source/references/processing.rst

doc/source/references/sensors.rst

doc/source/rendering.rst

doc/source/tutorials/examples/asv_homing_graph.py

doc/source/tutorials/examples/asv_homing_grid.py

doc/source/tutorials/index.rst