Rework the file upload docs

[javiereguiluz]

We've merged #19807 and now the /controller page shows the nice #[MapUploadedFile] attribute.

This new feature is fantastic ... but I wonder if it's a good idea to show it in /controller.

This is one of the most critical chapters of Symfony Docs and part of the "Getting Started" series. So, I think that showing this could be a bit overwhelming for readers, mostly newcomers.

Also, we have this page too, which now looks weird and outdated: https://symfony.com/doc/current/controller/upload_file.html

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So, this is what I propose:

• Update controller/upload_file.rst entirely:
  • Show the #[MapUploadedFile] attribute first
  • Think if we should still show the previous way. If yes, let's do it at the bottom of the article and more concisely
• Remove the #[MapUploadedFile] docs from controller.rst and only keep a tip mentioning it and linking to the doc about file uploads
• These changes should only be made on 7.1 branch

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What do you think?