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. 
you should put this in a different section of the page (to allow linking to it). And you should note that the dumper is only useful when using the extraction commands