Add release post for 2.15.0rc0

[mlflow-automation]

Be sure to fill in the contents

[github-actions[bot]]

Preview for 204f47e75a34a707d9f8af8e038a4942adcb8e99

• For faster build, the doc pages are not included in the preview.
• Redirects are disabled in the preview.

<img alt="Open in StackBlitz" src="https://developer.stackblitz.com/img/open_in_stackblitz.svg" />

[B-Step62]

Skipping because this is RC release.