fix(storage): update instructions for adding external S3 buckets to Amplify (#8289)

* fix(storage): update instructions for adding/integrating with external S3 buckets

* rewrite examples, add user identity ID config, add examples for configuring storage without Amplify backend

* address feedback, align some wording

* reorder guest and auth'd user examples, update backend code examples to use CDK bucket construct

* address feedback

* update callout for adding existing bucket and amplify storage together, revert callout on owners example section

* adjust headings, add heading before examples for better linking

Author: tiffanynwyeung