Merge branch '53596-mention-required-file-extension-yaml-include-docs' into 'master'

Update docs for include to mention required file extension

Closes #53596

See merge request gitlab-org/gitlab-ce!23048

doc/ci/yaml/README.md

@@ -1634,6 +1634,10 @@ rspec:
     - bundle exec rspec
 ```
 
+NOTE: **Note:**
+`include` requires the external YAML files to have the extensions `.yml` or `.yaml`. 
+The external file will not be included if the extension is missing.
+
 You can define it either as a single string, or, in case you want to include
 more than one files, an array of different values . The following examples
 are both valid cases: