document how to work with dcc.Loading loading state

@cdolfi
I'd like to improve how responsive my app feels. At the moment, I trigger a relatively large back-end request with a button that must complete before the graphs on my page, sequentially, enter and exit their respective loading states.

I would like to be able to manually set the is_loading field of the dcc.Loading component when I trigger this large back-end request but after having played with it I can't make this behavior happen.

It would be appreciated if an example could be provided for how to properly interact with this field of the dcc.Loading (and subsequently and nested dcc.*(loading_state={}) options).

Example of the clunky polish of the app that we're working on (Overview page):
https://sandiego-explorer.osci.io/overview

Note that the graphs are blank on the "overview" page until their respective sequential predecessors complete rendering and, I assume, the "is_loading" field is set on update.

Start with the dcc.Loading documentation and the loading_state and is_loading fields described in the issue, then compare the behavior with the linked Overview page. Document a working example for manually coordinating a large back-end request with dcc.Loading, including the nested dcc.* loading_state options. Done means a newcomer can follow the example to control the loading state and avoid sequential blank graphs.