Rewritten from scratch the chapter about installing Symfony

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #4122
I would change this sentence to this ("the" is missing, the `ref` label is not needed and "know" doesn't seem to be the right word to me):

```rst
Read the :ref:`book-creating-applications-without-the-installer` section to learn how to proceed.
```