Add Certificate glossary term (#7456)

* Add Certificate glossary term

Part of issue #5993

* Edits for clarity

_data/glossary/certificate.yaml

@@ -0,0 +1,11 @@
+id: certificate
+name: Certificate
+full-link: https://kubernetes.io/docs/tasks/tls/managing-tls-in-a-cluster/
+tags:
+- security
+short-description: >
+  A cryptographically secure file used to validate access to the Kubernetes cluster.
+long-description: >
+  Certificates enable applications within a Kubernetes
+  cluster to access the Kubernetes API securely. Certificates validate that
+  clients are allowed to access the API.