Removed Translation component documentation

[wouterj]

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.

[javiereguiluz]

Wouter, thanks a lot for this. It's a great work as always. I left some comments, but they are less important than they may look. Cheers!

[javiereguiluz]

I know this article explains something very specific ... but creating a new tiny article of just 44 lines is what we're trying to avoid recently. Couldn't we add this in a section (or sidebar) of the main translation article?

[wouterj]

I somewhat agree, but I don't think this matches our 80-20 rule. XLIFF formats have some specialized features, notes being one that is supported by Symfony. I don't think XLIFF notes belong to the 80% use cases of translation in a Symfony application. And if they do, it would target the translator more than a PHP developer (as notes are just for usage in the XLIFF editor AFAICS).

[wouterj]

Thanks @javiereguiluz. I've now updated the PR based on your suggestions. I've updated the docs to not talk much about message domains in af22e56 . I agree that's its a premium feature that might easily be overused and confusing, so we should not talk about it too much in the docs :)

[wouterj]

I've merged this into all versions of the docs and tweaked some docs that were moved to the main guide to follow previous changes between versions in c0f9bfa and 98da366