[WIP] Bootstrapped the Translation documentation

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.0+
| Fixed tickets | #952

### Todo

 - [x] Add documentation about domains

Finally got this document in a shape in which I can request a PR.

However, I'm still not sure if I like it now. Refering to a book article for more information seems wrong to me. Another thing I thought about is to move almost everything from the Translation book article to this component article. While I like that the most, I think it is huge change in the docs.

### After merging

 - Document loaders new in newer versions of Symfony