minor #12861 Removed Translation component documentation (wouterj)

This PR was squashed before being merged into the 3.4 branch (closes #12861).

Discussion
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Removed Translation component documentation

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.

Commits
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eb7158b Promote usage of a single translation domain
e3f4646 Move sidebar into a real section
2683376 Removed Translation component documentation

Author: wouterj

_build/redirection_map

@@ -369,3 +369,5 @@
 /contributing/community/other /contributing/community
 /setup/composer /setup
 /components/debug https://github.com/symfony/debug
+/components/translation https://github.com/symfony/translation
+/components/translation/usage /translation

bundles/override.rst

@@ -135,6 +135,6 @@ Translations
 
 Translations are not related to bundles, but to domains. That means that you
 can override the translations from any translation file, as long as it is in
-:ref:`the correct domain <using-message-domains>`.
+:ref:`the correct domain <translation-resource-locations>`.
 
 .. _`the Doctrine documentation`: http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/reference/inheritance-mapping.html#overrides

components/form.rst

@@ -54,7 +54,7 @@ support for very important features:
 The Symfony Form component relies on other libraries to solve these problems.
 Most of the time you will use Twig and the Symfony
 :doc:`HttpFoundation </components/http_foundation>`,
-:doc:`Translation </components/translation>` and :doc:`Validator </components/validator>`
+:doc:`Translation </translation>` and :doc:`Validator </components/validator>`
 components, but you can replace any of these with a different library of your choice.
 
 The following sections explain how to plug these libraries into the form