Add new message that API Mediation Layer is up

[balhar-jakub]

Describe your pull request here:
Adds message into the log that states that the API Mediation Layer is up.

List the file(s) included in this PR: troubleshoot-apiml-error-codes.md

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[github-actions]

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[github-actions]

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[github-actions]

🚀 Deployed on https://6630d5d6ef3bf8d3a7b009ad--zowe-docs-master.netlify.app

[janan07]

reviewed by doc squad