Doc upgrade checklist:

• old boilerplate at top and bottom of file has been removed
• Every example is independently runnable and is optimized for short line count
• no more plot() or iplot()
• graph_objs has been renamed to graph_objects
• fig = <something> call is high up in each example
• minimal creation of intermediate trace objects
• liberal use of add_trace and update_layout
• fig.show() at the end of each example
• px example at the top if appropriate
• v4upgrade: true metadata added
• minimize usage of hex codes for colors in favour of those in https://github.com/plotly/plotly.py-docs/issues/14