Rewritten from scratch the chapter about installing Symfony

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #4122
Let me explain: this note was originally located in the installation chapter that this PR is rewriting. As I felt that the note no longer fit in this chapter, I though about moving it to the next chapter, just after explaining which are the main Symfony directories and their purpose. But if you feel that this note is no longer necessary in the book, I'm OK with removing it.