Global state management: Update documentation

[umpox]

As part of https://github.com/sourcegraph/sourcegraph/issues/26655 we should add more documentation around using state:

• Managing different types of state:
  • Global state: shared across the application and should be stored in a single Zustand store.
  • 'Localized' state: Will typically be used for things like reusable components, and may change depending on some configuration, should be stored in a normal React context. Example: Setting a compact state for all FilteredConnection UI components in a tree.
  • GraphQL state: Data fetched from our backend API, should be stored in the Apollo cache.
  • Local state: Normal React state that should be isolated in components, e.g. useState
• FAQS
  • How to test
  • Common problems
  • ... etc

[fkling]

I still have a pending PR that would add documentation about zustand in general, but it's probably worth it to also have documentation not only about how to manage state, but also which global stores are available and what information they contain (e.g. the theme store). People who introduce new stores could document it on that page.