[Console] Added a cookbook entry on invoking other commands

This is a follow-up to an issue I've opened in [symfony/symfony](https://github.com/symfony/symfony/issues/12495). It describes how to find and execute other commands from a Symfony `Command`.
Symfony docs use a special title-casing. Even if I don't fully understand it, I think that this title should be:

```
Invoking Other Commands
```