Create documentation for how to update translations

[MirandaEcho]

Instructions with screenshots for translation checker to update translations for:

• General strings (how to find and update a translation)
• How to edit article translations, if needed

[benlk]

Ran out of time; relying on WPML docs to provide the screenshots and videos: https://docs.google.com/document/d/1srjlnJ6X4TrK2GBX8jnjghuj4TTF1sJKj2ei8y5Reg4/edit#

This doc is visible and editable to INN users, not yet to anyone else.

[MirandaEcho]

@benlk what's there looks good. Please spend the remaining time on fleshing out the section on how to update existing translations. The audience for this section is CIJN staff and the person checking the translations, so it needs to be as basic as possible and include screenshots where applicable. Thanks!!

[benlk]

Translation docs are updated.