Update documentation with new features

Missing examples and demos :

• slicing with step or negative band index
• dtype
• transform
• access values like image[10,10]
• convert xy to rowcol
• rasterio export (and import coming soon)
• summarize()
• OTB import helpers and env variables (to update)
• App.find_outputs()
• write multiple outputs with different extended filenames and dtypes
• Input and Output
• install_otb()
• simple multiproc example

Also it would be nice to remove special methods from the API docs since it is not useful for the end user