Add missing pages on documentation

[Hans5958]

Here are the current list of pages that is linked but needed writing at this moment. This list is not exhaustive, and others may add to the list by request or by editing this comment.

• Localization
  • [ ] Translating Scratch Addons (/docs/localization/translating-scratch-addons)
  • [ ] Localizing Background (/docs/localization/localizing-background)
  • [ ] Pushing and Pulling Messages (/docs/localization/pushing-and-pulling-messages)