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. 
I know that the original text also included it, but I wonder if *"some default Loaders"* should be *"some default loaders"*.