Add Middleware Onion to the docs

[inoas]

@markstory Could we have 2 graphics ... those at https://www.youtube.com/watch?v=DtH__Iz9Mjs at 19m32s and that at 20m00s ... maybe as an anim gif or 2 graphics... maybe one more injecting an onion ring.

That should help people understand better?

p.s.: I have been creating my own NotFoundMiddleware and found it amazing to stop the framework from loading early in a clean way - thanks for doing this - much better than DispatchFilters.

[markstory]

Where would you see that fitting in? Would it make sense as part of the intro to middleware?

[ravage84]

@markstory alternatively, somewhere around here. Or link from there to middleware (which is already the case, btw), where we place that image.

I would prefer an image, instead of a gif. Simply, because it can be better printed/exported to PDF.

[inoas]

@ravage84 good point regarding printing

I also thought about request-response circle... maybe there could be one image of the onion there and in the chapter about middleware we can use the same image and 2 more for explaining how to stop requests early and/or how to modify request/response while moving into/out of the onion layers.

[markstory]

Pull request open now.