Atmosphere as entity migration guide

[mate-h]

Objective

• Once PR Atmosphere as entity with transforms #23651 is merged we need a migration guide for breaking changes to Bevy's atmosphere.
• Post the release note changes as a new PR to review grammar/writing separately

Solution

• Write a migration guide detailing the breaking changes
• Adhere to a friendly non-technical tone
• Explain the new way of scaling the atmosphere with Transform

Out of scope

• Explain how to customize the apparent up axis (i.e. horizon's orientation) -> this should follow intuitively but maybe I should add this too? or maybe this belongs in a release note instead?

[github-actions]

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[ecoskey]

Could we add a sentence somewhere that makes it clear AtmosphereSettings is the component that enables the effect now? Most will probably be able to figure it out from context but it's also the kind of thing that's easily missed. Otherwise, looks good!