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. 
totally unrelated. The extractor does not care about the format of your translation files. It relies on the loader and dumper for it (it would be great to add a cookbook about adding extractors though)