Clarify linked CA requires Step CA Pro

Update documentation to clarify that linked CA functionality is now
available exclusively through Step CA Pro, not open-source step-ca.

Related PRs:
- smallstep/certificates#2554
- smallstep/cli#1559

PRO-331

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: tashian

certificate-manager/how-it-works.mdx

@@ -1,5 +1,5 @@
 ---
-updated_at: January 28, 2026
+updated_at: January 29, 2026
 title: Smallstep Certificate Manager How It Works
 html_title: Certificate Manager Architecture Guide
 description: Technical architecture of Certificate Manager. Understand components, workflows, and security model for enterprise PKI deployment planning.
@@ -348,7 +348,7 @@ sudo REQUESTS_CA_BUNDLE=$(step path)/certs/root_ca.crt \
 ACME works almost anywhere and is especially useful for issuing certificates to
 web servers for use with TLS. To issue certificates for internal names that
 aren't in public DNS the ACME server does need access to local DNS. It's common
-to use a linked CA or a network-local RA. Reference the
+to use a network-local RA (or a linked CA via [Step CA Pro](https://smallstep.com/product/step-ca-pro/)). Reference the
 [Certificate Manager docs](./acme/README.mdx) for more details.
 
 # Next steps

tutorials/acme-protocol-acme-clients.mdx

@@ -1,5 +1,5 @@
 ---
-updated_at: January 20, 2026
+updated_at: January 29, 2026
 title: Configure popular ACME clients to use a private CA with the ACME protocol
 html_title: Configure ACME Clients with step-ca Tutorial
 description: Configure popular ACME clients with step-ca. Support for Certbot, acme.sh, and other ACME implementations for automated certificate management.
@@ -25,7 +25,7 @@ If you run into any issues, please let us know [in GitHub Discussions][gh-discus
 ## Requirements
 
 - **Open source -** For `step-ca`, this tutorial assumes you have initialized and started up an instance using the steps in [Getting Started](../step-ca/getting-started.mdx). Additionally, you'll need to configure your CA with an ACME provisioner. Run `step ca provisioner add acme --type ACME`, and restart your CA. 
-- **[Smallstep Certificate Manager](../certificate-manager/acme/README.mdx) -** this tutorial assumes you have [created a hosted or linked authority](../certificate-manager/getting-started.mdx) and are running a local [ACME Registration Authority](../registration-authorities/acme-for-certificate-manager.mdx). 
+- **[Smallstep Certificate Manager](../certificate-manager/acme/README.mdx) -** this tutorial assumes you have [created a hosted authority](../certificate-manager/getting-started.mdx) (or a linked authority via [Step CA Pro](https://smallstep.com/product/step-ca-pro/)) and are running a local [ACME Registration Authority](../registration-authorities/acme-for-certificate-manager.mdx). 
 
 ## Overview
 

tutorials/kubernetes-acme-ca.mdx

@@ -1,5 +1,5 @@
 ---
-updated_at: September 17, 2025
+updated_at: January 29, 2026
 title: Integrate Kubernetes cert-manager with an internal ACME CA
 html_title: Kubernetes ACME CA Integration Tutorial
 description: Deploy ACME-enabled CA in Kubernetes clusters. Automate certificate management with native Kubernetes integration and step-ca.
@@ -22,7 +22,7 @@ using cert-manager's [ACME issuer](https://cert-manager.io/docs/configuration/ac
 ## Requirements
 
 - **Open source -** You have initialized and started up a `step-ca` ACME instance using the steps in [our ACME server tutorial](../step-ca/acme-basics.mdx).
-- **[Smallstep Certificate Manager](https://smallstep.com/certificate-manager) -** this tutorial assumes you have [created a hosted or linked authority](../certificate-manager/getting-started.mdx) and created an ACME provisioner with External Account Binding enabled.
+- **[Smallstep Certificate Manager](https://smallstep.com/certificate-manager) -** this tutorial assumes you have [created a hosted authority](../certificate-manager/getting-started.mdx) (or a linked authority via [Step CA Pro](https://smallstep.com/product/step-ca-pro/)) and created an ACME provisioner with External Account Binding enabled.
 - You'll need the root certificate PEM file for your CA.
 
 ### 0. Before you begin