ownCloud 8: Added doc for addTranslations()

Adds documentation for the new mechanism for loading translations from https://github.com/owncloud/core/pull/11638

Please review @DeepDiver1975 @LukasReschke @butonic 
Translation lookup stays the same.
The translations files are now a JSON file instead of PHP.
The difference is mostly on the JS side where translations need to be included early with `::addTranslations()` which will pre-load the translations with a JS file instead of lazy-loading it with a SYNC JS call (which blocks the UI)