Create route to get json schema to validate config

Fix visibility
Make compatible with php 7.2
Remove returing instance
Increase coverage
Reduce psalm info
Throw exception if system not defined
Increment coverage

Signed-off-by: Vitor Mattos <vitor@php.rio>

docs/API.md

@@ -992,6 +992,23 @@ For now only `deck_file` is supported as an attachment type.
 
 #### Request parameters
 
+| Parameter    | Type    | Description                                   |
+| ------------ | ------- | --------------------------------------------- |
+| system       | Integer | The system name. Example: trello              |
+
+#### Response
+
+Make a request to see the json schema of system
+
+```json
+{
+}
+```
+
+### GET /boards/import/config/system/{schema} - Import a board
+
+#### Request parameters
+
 #### Response
 
 ```json