Update docs to cover Angular Initializer changes in v7

[rdebeasi]

Related to #22674 (already closed) and #29386 (might close; I'm not sure).

What I did

I documented a breaking change in Storybook v7, per this recommendation. I also described the update I made to account for this change. If there's a better alternative - or if you'd prefer for me to remove that code snippet - just let me know!

I noticed that my editor removed some escaping and adjusted some whitespace. I think these changes are in line with the standards for this file, but I can back those changes out if they're incorrect.

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

(Not applicable - documentation only)

Manual testing

This change includes documentation only.

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Checklist for Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli/src/sandbox-templates.ts

• Make sure this PR contains one of the labels below:

  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

🦋 Canary release

This PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=<PR_NUMBER>

Greptile Summary

This pull request updates documentation to cover changes discussed in issue #22674, which was related to Angular's APP_INITIALIZER not firing in Storybook 7. The main changes include:

• Documenting a breaking change in Storybook v7 regarding Angular's APP_INITIALIZER
• Adding a code snippet demonstrating the recommended approach for using APP_INITIALIZER in Storybook 7+
• Updating the MIGRATION.md file to reflect these changes

Key points:

• The PR addresses a documentation gap for a known issue with Angular and Storybook 7
• It provides guidance for users on how to work around the APP_INITIALIZER issue
• The changes are documentation-only and do not require testing beyond review for accuracy
• The PR may help close issue [Bug]: Angular APP_INITIALIZER not firing at Storybook 8 #29386, which is related to the same APP_INITIALIZER problem in Storybook 8

This update should improve the migration guide for Angular users upgrading to Storybook 7 or 8, helping them understand and address potential issues with APP_INITIALIZER.

[greptile-apps]

LGTM

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