Revise README for maintenance mode announcement

[lesliecdubs]

What are you trying to accomplish?

Updated README to reflect maintenance mode status and migration guidance. View rendered content

Screenshots

Refer to diff

Integration

N/A

List the issues that this change affects.

N/A

[changeset-bot]

⚠️ No Changeset found

Latest commit: 5641be5

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[Copilot]

Pull request overview

Updates the repository README to announce that Primer ViewComponents has entered maintenance mode and provides migration guidance for internal GitHub and external consumers.

Changes:

• Adds a maintenance-mode warning section describing support scope and expectations.
• Provides migration guidance toward Primer React (internal) and fork guidance (external).
• Moves the prior “pre-1.0 active development” note into the Documentation section.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.