Amplify Libraries Swift - API GraphQL Upgrade guide - Add subscription examples

**Describe the content issue**:
The docs located here do not provide a subscription example. Update this doc with some examples for calling Amplify.APi.subscribe API with the API.swift compat layer.

We have some examples in our testing from this PR https://github.com/aws-amplify/amplify-swift/commit/0f9e1496ce6dba4da302bd8a1594a85c8d2982c0#diff-6e35472267098df96f6f8accc728d62df90c4fcc65da973baba9291088ff1860

**URL page where content issue is**:
https://docs.amplify.aws/lib/graphqlapi/upgrade-guide/q/platform/ios/

Start at the linked iOS GraphQL API upgrade-guide page and inspect the subscription examples in the referenced amplify-swift testing commit. Add examples showing calls to Amplify.API.subscribe through the API.swift compatibility layer; the work is done when the guide documents subscription usage clearly alongside the existing upgrade content.

Written by the indexing model from the issue text.