Add documentation for i18n in Hilla

[marcushellberg]

We should have documentation that covers the basic internationalization of Hilla apps:

• Translating strings
• Translating validation messages
• Formatting dates and numbers

[platosha]

Should we settle on using @lit/localize? Seems to be a good fit for the enterprise apps, since it provides tooling for extracting messages, and supports XLIFF. Any other alternatives?

[marcushellberg]

I think that would make sense, it's the official solution for Lit in any case.