Additional docs

### Salt for page previews and media files

URLs of page previews and media files contain a hashed token that should be hard to guess. The tokens are based on the page URI or file ID and are authenticated with a salt value. In the way Kirby uses this salt value, it should ideally be a secret. By default, Kirby uses the filesystem path of the content folder. You can define your own salt that will be used instead:

\```php
return [
  'content' => [
    'salt' => '...'
  ]
];
\```

You can also dynamically generate a salt based on the model that needs a token:

\```php
return [
  'content' => [
    'salt' => function ($model) {
      return '...';
    }
  ]
];
\```

<since v="5.0.0">
The `$model` value is set to the `File` object when generating a media token for a file. For page preview tokens, `$model` is always passed as `null`. Your callback should return a fixed model-independent salt in this case.
</since>

<alert>
Setting the option to `true` allows all visitors to download the uploaded file originals in full quality and resolution and with all metadata that the file carries. This can be a copyright, privacy or information security concern. Only set the option to `true` if none of your files require access protection.
</alert>