Add summary of changes for v2.15

[janan07]

Describe your pull request here: This PR add the summary of changes for v2.15

List the file(s) included in this PR: v2_15_0.md

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[github-actions]

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[github-actions]

📁 The PR description is missing the file name(s) for the updated content. List all the files included in this PR so this information displays in our Zowe Docs GitHub Slack channel.

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[github-actions]

📌 The subject area label is missing. Add an area: label so we know what your content is about.

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[github-actions]

🚀 Deployed on https://65e05c8c6fd201f21e34305a--zowe-docs-master.netlify.app

[balhar-jakub]

Seems to be good for API ML.

[anaxceron]

@janan07 -- This content is missing. Wasn't sure if you were waiting to add this yourself later on, but wanted to point this out just in case.