Added cookbook to show how to make a simple upload

| Q             | A
| ------------- | ---
| Doc fix?      | No
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #2346

This documentation refers to the ticket #2346, the second point saying:
```
2) Have a cookbook article that talks about uploading in Symfony, but nothing about Doctrine. This would include using a Form, validation, and moving the uploaded file in the controller.
```

Not sure if it was wise to put it in the cookbook/controller section. Feel free to tell me if I should put it elsewhere.
This doesn't work (you cannot use the ``doc`` role to lik to a certain section). Instead use something like this:

```rst
:ref:`form themes <cookbook-form-customization-form-themes>`
```