OSDOCS-15490 created the about section for cert-manager plugin

Author: wgabor0427

modules/zero-trust-manager-cert-manager-upstream-authority.adoc

@@ -0,0 +1,11 @@
+// Module included in the following assemblies:
+//
+// * security/zero_trust_workload_identity_manageer/zero-trust-manager-overview.adoc
+
+:_mod-docs-content-type: CONCEPT
+[id="zero-trust-manager-cert-manager-upstream-authority_{context}"]
+= About the cert-manager upstream authority plugin
+
+The cert-manager plugin for the SPIRE server is designed to automate the management of the SPIRE server's intermediate signing certificates by integrating with cert-manager in a Kubernetes environment. The cert-manager plugin enables the SPIRE server to dynamically request and receive intermediate signing certificates from cert-manager.
+
+When a SPIRE server needs a new certificate, the cert-manager plugin creates a `CertificateRequest` custom resource in the configured Kubernetes namespace which contains the Certificate Signing Request (CSR) generated by the SPIRE server. The cert-manager plugin processes the `CertificateRequest` and an associated `Issuer` signs the CSR. The signed intermediate certificate and the full Certificate Authority (CA) bundle are made available in the `CertificateRequest` status. These signed credentials are made available to the SPIRE server to be used as its upstream signing authority.