Chatbot tutorial

[florimondmanca]

Is your feature request related to a problem? Please describe.
The current quickstart guide is arguably just a pile of code, and I feel we can do better.

Describe the solution you'd like
Let's keep the hello world example from the quickstart guide, and replace the rest with a step-by-step tutorial of building a chatbot server, from design to testing.

Describe alternatives you've considered

• Keep the current quickstart. Not informative nor inspiring enough.
• An echo WebSocket server. Pointless / not representative of the real world.

Implementation ideas

• ChatterBot.
• Our built-in WebSocket stack.
• Create a WS client using the websockets library.
• Showcase the usage of providers.
• Write some tests with pytest.
• Perhaps deploy to Heroku or something?

Additional context
A lot of stuff in the docs was written early and I think we need to brush things up a bit. This is part of that!