Split guide "Commands" into two guides

Nobody has claimed this yet.

I'm not 100% sure whether it is a good idea but think so.

We currently have one guide for [Commands](http://developer.piwik/guides/piwik-on-the-command-line) which handles how to use the console and one tiny section how to generate a command. This section is tiny as we have not merged the blog post yet which will be done in refs #44

The console is a crucial tool for developers as they will use it to generate all sort of things, to activate plugins, to clear caches, to enable the developer mode etc. Maybe we can move the current guide to `Getting started` between `Setting up` and `Distributing your plugin`? The title could be "Piwik on the command line". We do link to this from many blog posts already. Then add a new guide named "Commands" under `Plugin Basics` where it is currently positioned as well. This guide will only explain how to generate and develop new commands.

Start with the current Commands guide linked in the issue and inspect the guide navigation around Getting started and Plugin Basics. Review refs #44 for the pending command-generation material. Done means the console material is positioned as “Piwik on the command line” and a separate Commands guide covers generating and developing commands.

Written by the indexing model from the issue text.