Update Settings/Preferences documentation #11194

As the codebase evolves, it makes some of our documentation outdated. The current Settings documentation is one of them, and we need to update it to make sure contributors understand what is the current expectations when adding a new preference to the app.

Docs to update:

• Settings Architecture
• Preference Migration Guide

Nice to have

• The :core:configstore is currently not being used, but could get some attention by having it documented.