add doc about form helpers

docs/utils/form.md

@@ -0,0 +1,20 @@
+# Form
+
+## File picker
+
+Murph provides a file picker using the file manager system: `App\Core\Form\FileManager\FilePickerType`. It allows you to pick an existing file or upload it. After saving, the file is previewed if possible.
+
+## Collection
+
+When you need to manage a collection in a form, you can use `App\Core\Form\Type\CollectionType` with these options:
+
+
+| Option                 | Type     | Default  | Description                                                   |
+| ------                 | ----     | -------  | -----------                                                   |
+| `collection_name`      | `string` | `null`      | Unique name of the collection                                 |
+| `label_add`            | `string` | `Add`    | Unique name of the collection                                 |
+| `label_delete`         | `string` | `Delete` | Unique name of the collection                                 |
+| `template_before_item` | `string` | `null`      | A template included before an existing itam of the collection |
+| `template_after_item`  | `string` | `null`      | A template included after an existing itam of the collection  |
+
+...and and all options of a symfony collection type.

mkdocs.yml

@@ -99,6 +99,7 @@ nav:
       - File attribute: utils/file_attribute.md
       - Mail notifier: utils/mail.md
       - Slug: utils/slug.md
+      - Form: utils/form.md
     - "A/B Testing": abtesting.md
     - Users: users.md
     - Tasks: tasks.md