Add i18n test helper documentation.

[sainthkh]

Summary

Fixes #44975.

In #44580, new test helpers are created to be used with FormattedMessage. But documentation wasn't updated with that PR.

So, I've written one for that.

You can read the rendered version here.

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

• This was checked for cross-browser compatibility, including a check against IE11
• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[sainthkh]

When you (Kibana team) are ready to handle this, please leave me a comment. I'll come back to handle the conflicts and bugs.

[elasticmachine]

Pinging @elastic/kibana-stack-services (Team:Stack Services)

[maryia-lapata]

The documentation looks pretty good for me. Thank you for adding it.

[Bamieh]

LGTM! Thank you for submitting this

[Bamieh]

@elasticmachine merge upstream

[lukeelmers]

@elasticmachine merge upstream