add Amplify.configure migration docs for config customizations

[cwomack]

Describe the content issue:
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A customer reported the current migration docs don't have a section to migrate a manual Amplify configuration. The Amplify.configure method in v5 allows to override the default storage mechanism, set the AuthFlowType used by authentication flows, set a custom userpool endpoint, etc.

URL page where content issue is:
amplify-js repo Issue #13740