Add link to keep billing account active

[mijeffer]

No description provided.

[learn-build-service-prod]

Learn Build status updates of commit 9359300:

⚠️ Validation status: warnings

File	Status	Preview URL	Details
articles/cost-management-billing/toc.yml	⚠️Warning		Details

articles/cost-management-billing/toc.yml

• Line 39, Column 11: [Warning: file-not-found - See documentation] Invalid file link: 'understand/keep-billing-account-active.md'.

For more details, please refer to the build report.

Note: Your PR may contain errors or warnings or suggestions unrelated to the files you changed. This happens when external dependencies like GitHub alias, Microsoft alias, cross repo links are updated. Please use these instructions to resolve them.

[v-regandowner]

@mijeffer - Microsoft authors and contributors need to make content contributions through the private repository so the content can be staged and validated by the current validation rules.

After you make your changes in the private repo, review the articles on staging, and fix any validation issues, you can add the sign-off comment to let PR reviewers know that the updates are ready to be merged.

Read about authors working in the public repo for more information.

If you need help getting set up to work in the private repo, see Get started contributing to the Microsoft Learn platform.

We'll close this PR. Thank you.