Translation API guide

[mlewand]

📝 Provide a description of requested docs changes

In ckeditor/ckeditor5-utils#334 we introduced plural versions for translations.

The PR made attempt to attempt to explain the concept in API docs, but this won't cut it.

What we need here is a complete guide on how to add/work with translations, including a section how plural forms are handled.

Some points that I'd like to see covered:

• Adding translations to an existing build (so the function addTranslations( language, translations ) from docs)
• Translation API usage in the docs (very simple example, how t() is used)
• Plural forms

It should contain cross link to translation contribution section.

[mlewand]

Couple more notes:

https://github.com/ckeditor/ckeditor5-utils/blob/14d29a66792419fb4d59eb8869d3b438d2b15aec/src/translation-service.js#L34-L50 - this fragment should be covered by the "Plural forms" section and replaced with a link to the guide. The first code listing is incomplete (doesn't have the getPluralForm arg).