Write Sentry docs for standalone app start, extend api and app start sample rate APIs [Flutter]

[buenaflor]

Description

Write documentation for the new standalone app start feature and the app start sample rate API for Flutter.

Scope

• Document the enableStandaloneAppStartTracing option (feat: Send standalone app start transactions [Flutter] #3634)
• Document the app launch span sample rate API (API for setting app launch span sample rate [Flutter] #3635)
• Document the new extend app start api
• Explain the new standalone transaction structure (app.start.cold / app.start.warm)
• Provide migration guidance for users moving from the old attached-to-navigation behavior

Reference

Mirror of getsentry/sentry-java#5261

[linear-code]

DART-354

[buenaflor]

Completed in getsentry/sentry-docs#18903