[SDC-29853] Fix Frameworks Code Snippets

[ilber]

In addition to the fixes on the code snippets for Flutter, React-Native, Cordova, Capacitor and Titanium I have also updated the DataCaptureContext initialization pattern across Flutter, Capacitor, Cordova, and React Native docs

This MR updates the documentation to reflect the new DataCaptureContext API, where initialization is done once via an async call and the context is accessed afterwards through a shared instance.

Code snippets

• Replaced DataCaptureContext.forLicenseKey(...) with await DataCaptureContext.initialize(...) across all affected get-started and barcode-generator guides
• Updated all subsequent references to the context variable (context, dataCaptureContext) to use DataCaptureContext.sharedInstance