Flowchart for better proxy plugin documentation

[eherrerosj]

The objective of this issue is to ease the understand of the process for building new proxy plugins.

Is your feature request related to a problem? Please describe.
I have to go to check the internal source code every time I need to figure out in what order the proxy functions are executed.

Describe the solution you'd like
Having a flowchart explaining the sequence of functions that requests and responses go through.

Example
Scrapy, for example has this flowchart that results to be super useful to devs:

[abhinavsingh]

@eherrerosj Thank you for raising this concern. Indeed a better diagram will help put things into perspective. Currently there is an outdated ASCII art and bunch of ipynb tutorials. I'll look into it and come up with something useful.