Add documentation for credentials to website (#1356)

website/data/docs-nav-data.json

@@ -123,6 +123,18 @@
             "title": "Auth Methods",
             "path": "concepts/domain-model/auth-methods"
           },
+          {
+            "title": "Credentials",
+            "path": "concepts/domain-model/credentials"
+          },
+          {
+            "title": "Credential Libraries",
+            "path": "concepts/domain-model/credential-libraries"
+          },
+          {
+            "title": "Credential Stores",
+            "path": "concepts/domain-model/credential-stores"
+          },
           {
             "title": "Groups",
             "path": "concepts/domain-model/groups"