docs: document secret environment variables

Author: isshaddad

docs/deploy-environment-variables.mdx

@@ -30,6 +30,15 @@ We deploy your tasks and scale them up and down when they are triggered. So any
   locally.
 </Note>
 
+### Secret environment variables
+
+When creating an environment variable, you can mark it as a **Secret**. Secret values are hidden in the dashboard and cannot be viewed after creation.
+
+<Warning>
+  Marking a variable as a Secret is irreversible and can only be done when creating the variable. To
+  change this setting, you must delete the variable and create a new one.
+</Warning>
+
 ### Editing environment variables
 
 You can edit an environment variable's values. You cannot edit the key name, you must delete and create a new one.