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. 
this new line will not be detected by Sphinx. You either have to add an empty line between the 2 sentences to create 2 paragraphs, or you can simply remove the line break.