Add banner for migration

[sjaiswalh]

No description provided.

[hashicorp-cla-app]

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[hashicorp-cla-app]

Thank you for your submission! We require that all contributors sign our Contributor License Agreement ("CLA") before we can accept the contribution. Read and sign the agreement

Learn more about why HashiCorp requires a CLA and what the CLA includes

_{Have you signed the CLA already but the status is still pending? Recheck it.}

[Copilot]

Pull request overview

This PR adds a migration notice banner to inform users about the upcoming transition from Hermes to a SharePoint-backed system. The banner replaces the previous "What's new in Hermes" feature announcement section.

Changes:

• Replaced feature announcement content with migration notice
• Added migration timeline and read-only mode information
• Included link to new SharePoint-backed Hermes system

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