Docs updates 📚

[jhuleatt]

Now that we better support lazy loading, as well as React's preload/render-as-you-fetch pattern, it is time to update the docs.

Preview here

Changes

• Add init instructions to usage docs
• Update quickstart samples to promote lazy loading and adopt React render-as-you-fetch pattern
• Update samples to use ReactDOM.createRoot
• Decide whether we still want the quickstart, and, if so, update it with some render-as-you-fetch explanation (note: I decided against mentioning this in the quickstart, and instead added it to the usage docs)
• Update the "common use cases" section
  • Read data from Firestore only when a user is logged in
  • Log page views with Analytics
  • How to solve Warning: App triggered a user-blocking update that suspended. with useTransition
    - [ ] Something with Remote Config (note: going to save this for another day)
  • Log page views with Google Analytics for Firebase
  • document useObservable for advanced use cases
• Update "What is ReactFire" in readme to emphasize lazy loading
• Double check that CONTRIBUTING.md is up to date

Stretch Goals

- [ ] Auto-generate reference docs from source

[jamesdaniels]

How about "status: experimental". We should also add that "we will aim to be API Stable, however"