[Translator] Document intl format

[Nyholm]

We should document how to use intl format.

See related PR: symfony/symfony#27399

Some description of the format here:
https://messageformat.github.io/messageformat/page-guide

[javiereguiluz]

Thanks for opening this issue Tobias. Hopefully you'll find sometime to document this new feature yourself ... because I'm afraid I don't understand it well enough to document it. Thanks!

[Nyholm]

Yes. That is the plan. But in case I forget or are too slow I thought it would be a good thing to write it down.

I'll write on this issue when I begin with this feature.
Anyone: feel free to start and we'll complete it together.

[nicolas-grekas]

A nice intl+icu fiddle we could link in the doc!
https://format-message.github.io/icu-message-format-for-translators/

[nicolas-grekas]

Should also tell about symfony/symfony#28952

[javiereguiluz]

Fixed by the great #11523 PR.