docs: Document usage of SentryWidget

[vaind]

Description

SentryWidget enables multiple features in the Flutter SDK. I've seen multiple instances where people don't have them in their code. From the look at the docs, it's only present when talking about enriching data (e.g. screenshots). Instead, I believe we should move it to Getting Started as well as the onboarding wizard flow so that it's something people set up early enough when they're setting up Sentry.

Additionally, we should mention SentryWidget on all the pages that describe the functionality that require it, e.g. replay, screenshots, user-interaction tracking, etc.