Add tutorials to user documentation

[ArthurDanjou]

Provide step-by-step practical guides for new framework users.

Here is a non-exaustive list of tutorials :

• "Quick Start" tutorial — run a simple training with MultiKrum
• "Implement an aggregator" tutorial — add a custom aggregator
• "Implement an attack" tutorial — create a custom Byzantine attack
• "Orchestrator and experimentation" tutorial — set up an experiment campaign
• "Reproduce an experiment" tutorial — reproduce paper results (NIPS 2017, ICML 2018)
• "Understanding the Model wrapper" tutorial — zero-copy flat tensor views of parameters and gradients
• "Running an experiment from A to Z" tutorial — create a simulation, define an experiment, and use the orchestrator