Add templated rules example

docs/examples/templates.hcl

@@ -0,0 +1,25 @@
+# Examples of rules using templates.
+
+# Any alert using non-zero `for` field must also
+# have a label named `alert_for` with the value
+# equal to the `for` value.
+# Alert defined as follows:
+# - alert: service_down
+#   expr: up == 0
+#   for: 5m
+# would fail this check, but this version wouldn't:
+# - alert: service_down
+#   expr: up == 0
+#   for: 5m
+#   labels:
+#     alert_for: 5m
+rule {
+  match {
+    for = "> 0"
+  }
+
+  label "alert_for" {
+    required = true
+    value    = "{{ $for }}"
+  }
+}