Clean up Renderer API #2191
Description
As discussed in #2122 we need to revisit the Renderer API, primarily because of the _figure_data method which is currently considered internal API but is commonly useful in certain cases. Additionally we should revisit the API around deploying to bokeh server, e.g. the server_doc method and potentially also consider adding something equivalent to a show method which is useful to open a plot outside of a notebook context.