dhi: add migration tip (#23327)

<!--Delete sections as needed -->

## Description

Added migration tip to inform existing Docker Hub users that they can
keep using existing workflows and tools when migrating from other images
on Docker Hub.

https://deploy-preview-23327--docsdocker.netlify.app/dhi/get-started/

## Related issues or tickets

<!-- Related issues, pull requests, or Jira tickets -->

## Reviews

<!-- Notes for reviewers here -->
<!-- List applicable reviews (optionally @tag reviewers) -->

- [ ] Editorial review

---------

Signed-off-by: Craig <craig.osterhout@docker.com>

_vale/config/vocabularies/Docker/accept.txt

@@ -12,6 +12,7 @@ autolock
 Azure
 Azure AD
 bootup
+Bitnami
 Btrfs
 Bugsnag
 BuildKit

content/manuals/dhi/get-started.md

@@ -12,6 +12,19 @@ This guide shows you how to go from zero to running a Docker Hardened Image
 (DHI) using a real example. While the steps use a specific image as an
 example, they can be applied to any DHI.
 
+> [!TIP]
+>
+> You can keep using the same tools and workflows you already know when moving
+> to DHI from other images on Docker Hub, such as Bitnami public catalog images.
+> Note that [Bitnami announced](https://github.com/bitnami/charts/issues/35164)
+> that its public catalog images will no longer be available after September 29,
+> 2025.
+>
+> In most cases, migrating is as simple as updating the image reference in your
+> configuration or commands. Start with this guide, then see the [migration
+> guide](./how-to/migrate.md) for more details and examples.
+
+
 ## Step 1: Sign up and subscribe to DHI for access
 
 To access Docker Hardened Images, your organization must [sign