☂️ Improve guide for localization and use of `l10n.yml` file

[Craftplacer]

What information is missing?

l10n.yml is poorly documented and has to be pieced together by reading the Internationalization User Guide or some information that can't be found at all (e.g. preferred-supported-locales, an option added by a contributor).

How would you like us to fix this problem?

Make a dedicated page or update the existing page for l10n/i18n, which includes an easily readable list/table of the available options that the developer can set in order to get the most out of the localizations provided by flutter_gen.

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Include the following when updating:

• [x] https://github.com/flutter/website/issues/7627
• [ ] https://github.com/flutter/website/issues/2885
• [ ] https://github.com/flutter/website/issues/3998
• [x] https://github.com/flutter/website/issues/6424
• [x] https://github.com/flutter/website/issues/7627
• [x] https://github.com/flutter/website/issues/8503