While translation is one of the components that is used by other applications quite a lot, most of the components docs was about general translator usage (that also applies to usage in the framework). Setting up the translation component isn't that difficult.

This PR moves all relevant docs from the components doc to the main guide. Much of this was duplicated in both, so that proves that this move is a usefull thing to do. 2 articles have been completely removed: Creating your custom message formatter and format. A custom message formatter seems really advanced usage. The Translation component provides so many formats that I think creating a custom format is also quite advanced. If it turns out this is not true, we can rest assured that they still exist in GIT history so we can quickly add them back.

Please note that the main translator guide is a really ugly structured document. However, let's only do the rewriting in Symfony 4.3+, as lots has changed (flex, ICU message format, etc) between 3.x and 4.x.