Alire documentation overhaul

[Fabien-Chouteau]

I got some feedback on the document and I think we can improve it. We have a lot of good content but I feel like the layout could be better.

I propose the following outline:

Introduction

What is Alire? Why it exists? Why may I want to use it?

Getting Started

Installation

Get and build the hello crate

Your first crate

Toolchains

Alire User's Guide

Alr commands

Manifest format specifications

Publishing a crate

User-facing changes log

Frequently Asked Questions

[mosteo]

Seems sensible to me. Certainly what we have now is old from the early days IIRC.