Add deprecation message for CDKTF

[robin-norwood]

What

Add deprecation message for CDKTF.

• 🔗 Companion PR to remove tutorials and redirect to docs index page.
• 🔗 Companion PR for dev-portal.

🔍 Build preview.

Note: So far, only the latest version, will backport once we're good on the text.

Why

CDKTF is being deprecated Dec 10th, 2025.

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Merge Checklist

If items do not apply to your changes, add (N/A) and mark them as complete.

Pull Request

• Description links to related pull requests or issues, if any.

Content

• You added redirects to content/terraform-docs-common/redirects.jsonc for moved, renamed, or deleted pages across all affected versions. Refer to Redirects for examples and guidance.
• Links to related content where appropriate (e.g., CLI, language, API endpoints, permissions, etc.).
• Pages with related content are updated and link to this content when appropriate.
• Sidebar navigation files have been updated for added, deleted, reordered, or renamed pages.
• New pages have metadata (page name and description) at the top.
• New images are 2048 px wide. They have HashiCorp standard annotation color (#F92672) and format (rectangle with rounded corners), blurred sensitive details (e.g. credentials, usernames, user icons), and descriptive alt text in the markdown for accessibility.
• New code blocks have the correct syntax and line breaks to eliminate horizontal scroll bars.
• UI elements (button names, page names, etc.) are bolded.
• The Vercel website preview successfully deployed.

Reviews

• I or someone else reviewed the content for technical accuracy.
• I or someone else reviewed the content for typos, punctuation, spelling, and grammar.

[github-actions]

Vercel Previews Deployed

Name	Status	Preview	Updated (UTC)
Dev Portal	✅ Ready (Inspect)	Visit Preview	Wed Dec 10 14:42:32 UTC 2025
Unified Docs API	✅ Ready (Inspect)	Visit Preview	Wed Dec 10 14:38:24 UTC 2025

[github-actions]

Broken Link Checker

No broken links found! 🎉

[rkoron007]

Approving to unblock - left a suggestion for a style tweak!