Document different approach to disabling stan.yml

Author: mpilgrem

.github/workflows/stan.yml

@@ -1,15 +1,18 @@
+# This workflow has been disabled on the Stack repository because Stan does not
+# yet support GHC >= 9.0. See: https://github.com/kowainik/stan/issues/423.
+#
+# On enabling a disabled workflow, see:
+# https://docs.github.com/en/actions/managing-workflow-runs/disabling-and-enabling-a-workflow#disabling-a-workflow
+#
 name: stan
 
-# Disabled because Stan does not yet support GHC >= 9.0. See:
-# https://github.com/kowainik/stan/issues/423
-
-# on:
-#   pull_request:
-#   push:
-#     branches:
-#     - master
-#     - stable
-#     - rc/**
+ on:
+   pull_request:
+   push:
+     branches:
+     - master
+     - stable
+     - rc/**
 
 jobs:
   build: