React: Restore webpack as a dependency, updated webpack 5 docs

[patrick-mcdougle]

Issue: #17192 (comment)

What I did

Restored webpack as a dependency with a version string that's a little more flexible (matches the strictest overlap of the dependencies peers). As this could result in a separate webpack version to be installed, I also added more to the migration guide about forcing a webpack 5 resolution if the user is attempting to use builder/manager webpack 5 and is running into issues.

How to test

• Is this testable with Jest or Chromatic screenshots?
• Does this need a new example in the kitchen sink apps?
• Does this need an update to the documentation?

If your answer is yes to any of these, please make sure to include it in your PR.

[nx-cloud]

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[shilman]

LGTM!