Add documentation for #[MapUploadedFile] attribute

[renedelima]

Closes #19802

[renedelima]

@OskarStark I applied your suggestions, fixed the Code blocks check, and made some other changes. The PR is now ready for review.

[mttsch]

Should the attribute also be added to https://symfony.com/doc/7.1/reference/attributes.html#httpkernel?

[renedelima]

Should the attribute also be added to https://symfony.com/doc/7.1/reference/attributes.html#httpkernel?

The link was added.

[javiereguiluz]

Renê, thanks a lot for adding these docs ... and congrats on your first Symfony Docs contribution 🎉

What a beautiful contribution you did to Symfony code (and docs too!). Tomorrow I'll publish a "New in Symfony 7.1" post dedicated to this.

Note: while merging I did some tweaks and rewords. See 846cbae. It looks like I changed a ton of things, but it's not true:

• I changed the order to display first the simple example with no constraints
• I changed the style of the controllers to show a standard controller extending AbstractController instead of an invokable controller (your original example is perfectly correct, but this article is one of the basic chapters of Symfony Docs, so I prefer to keep the same basic style as in the rest of the page)
• Other minor tweaks

So, most of your contribution stays. Thanks!