Document the SPM migration playbook for end users

Once RNSentry publishes a `Package.swift` and the React Native ecosystem supports SPM autolinking, end users will need clear guidance on:

* Whether to use CocoaPods or SPM for a given app.
* How to migrate an existing CocoaPods-based app to SPM.
* What the support matrix looks like (which RN versions support each path).
* What to do with Expo apps, brownfield iOS apps, and edge cases.

Without explicit guidance, users will hit the transition in an ad hoc way and surface confused support tickets to us.

Start by reviewing the planned RNSentry Package.swift and the React Native ecosystem's SPM autolinking support. Document the CocoaPods-versus-SPM choice, migration steps, RN-version support matrix, and guidance for Expo, brownfield iOS apps, and edge cases; the playbook is done when these paths are explicit for end users.

Written by the indexing model from the issue text.