Add instructions on how translations function into Translation management page

[StephenOTT]

pages/settings/translations

Add some instructions on what is shown on the page, and how to add new translations, how to override translations, and how to get/find the translation key for the field.

[baddlan]

Added translation instructions in SDK docs. https://github.com/DigitalState/Cms/blob/develop/translations-howto.md