Add WidgetRef documentation

[snapsl]

Make riverpod onboarding easier.

WidgetRef is a core functionality of riverpod. Currently the documentation mention WidgetRef in various places but never introduces it. (link, link) It should be given its own starting page to showcase its usage.

The documentation should contain:

• what is WidgetRef (the WidgetContext for riverpod)
• accessing the state of a provider
  • ref.watch() /ref.read()
• accessing the notifier of a provider
• listen to changes using .listen()
• maybe other methods (e.g. .exists(), ... )

If needed, I can contribute to the docs.

[rrousselGit]

There's https://riverpod.dev/docs/concepts/reading already

[snapsl]

I knew I had missed something.😅