docs: add internationalization (i18n) guide

[Gaic4o]

Background

This guide explains where i18n-related code can be placed in an FSD project.

It focuses on the responsibilities of i18n setup, translation resources, locale, and language switching, including how shared/i18n, app/i18n, entities, and features can be used, as well as examples of organizing translation resources based on project size and usage scope.

Changelog

Closes #171

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[Solant]

A bit of feedback from my end:

• nit: i18n setup looks a bit React-specific, especially with i18n instance passing, which might be a bit disruptive for other frameworks like Vue, so I'm not sure if this step is really necessary
• I like the point of not creating i18n as an entity; I would even ban this approach right away
• While I don't really like storing messages and i18n config in shared/i18n, it is an understandable approach. but using i18n as a slice segment outside shared is a bit worrying, as we try to keep custom segments to the minimum (we even had assets segment removed in favor of /ui/assets/), so I think that we should not make an exception there and use <slice>/ui/messages or <slice>/ui/i18n for localized messages (just like you did with /ui/messages.ts)
• I don't really like the example of User entity that uses Locale. I think it would be better to make an example of User being part of shared/api or shared/auth, for example

But overall I like this article; great job!