[2025-04] Update Buyer Journey API docs

[avocadomayo]

Background

Update 2025-04 Buyer Journey API docs with guidance and examples to show a warning banner in the Checkout and Accounts editor when "block progress" has not been granted to the extension by the merchant.

Solution

(Describe your solution, why this approach was chosen, and what the alternatives/impacts may be)

Checklist

• I have 🎩'd these changes
• I have updated relevant documentation

[github-actions]

We detected some changes in packages/*/package.json or packages/*/src, and there are no updates in the .changeset directory. If the changes are user-facing and should cause a version bump, run yarn changeset to track your changes and include them in the next release CHANGELOG. If you are making simple updates to repo configuration, examples, or documentation, you do not need to add a changeset.