documentation updates

GraphicsScene/GraphicsScene.py

GraphicsScene/mouseEvents.py

__init__.py

documentation/source/apireference.rst

documentation/source/graphicsscene/graphicsscene.rst

documentation/source/graphicsscene/hoverevent.rst

documentation/source/graphicsscene/index.rst

documentation/source/graphicsscene/mouseclickevent.rst

documentation/source/graphicsscene/mousedragevent.rst

documentation/source/how_to_use.rst

documentation/source/introduction.rst

graphicsItems/PlotItem/PlotItem.py

imageview/ImageView.py

widgets/PlotWidget.py