Add migrating between resources section

[maciejporebski]

Add a section to cover behaviour experienced when attempting to bind the same hostname to multiple SWA resources.

[prmerger-automator]

@maciejporebski : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[Copilot]

Pull Request Overview

This PR adds documentation to explain the behavior and limitations when migrating custom domains between Azure Static Web Apps instances, specifically addressing the constraint that domains can only be bound to one resource per slice.

• Adds a new section explaining domain binding limitations between Static Web Apps instances
• Provides guidance on identifying which slice a resource belongs to
• Offers solutions for migrating domains between instances in the same slice

[learn-build-service-prod]

Learn Build status updates of commit d22a886:

✅ Validation status: passed

File	Status	Preview URL	Details
articles/static-web-apps/custom-domain.md	✅Succeeded

For more details, please refer to the build report.

[learn-build-service-prod]

Learn Build status updates of commit 0057091:

✅ Validation status: passed

File	Status	Preview URL	Details
articles/static-web-apps/custom-domain.md	✅Succeeded

For more details, please refer to the build report.

[v-dirichards]

@v1212

Can you review the proposed changes?

Important: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team