Translation custom loaders

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #4156

This PR adds a documentation section for creating custom loaders and dumpers to support other formats. 
Using `::` at the end of this paragraph would make the code listing be interpreted as PHP code. Maybe its better to create a proper code listing:

```
[...]
message. A translation file would look like this:

.. code-block:: text

    (welcome)(Bienvenido)
    (goodbye)(Adios)
    (hello)(Hola)
```