Add the action stub

This is required for the migration of the codeql-action to an
immutable action.

We can delete this after the migration is done, or just keep it
so people have an extra way to understand how to use the action.

action.yml

@@ -0,0 +1,11 @@
+name: 'CodeQL: Stub'
+description: "Stub: Don\'t use this action directly. Read [the documentation](https://docs.github.com/en/code-security/code-scanning/introduction-to-code-scanning/about-code-scanning-with-codeql) instead."
+author: 'GitHub'
+runs:
+  using: 'composite'
+  steps:
+    - name: 'Stub'
+      run: |
+        echo 'This is a stub. Read [the documentation](https://docs.github.com/en/code-security/code-scanning/introduction-to-code-scanning/about-code-scanning-with-codeql) instead.'
+        exit 1
+      shell: bash