Document file_extension in the secure files API

What does this MR do and why?

The project-level secure files endpoints return a file_extension field, and none of the four example responses on this page shows it. This adds it to each of them.

I placed it after metadata so the examples keep the order API::Entities::Ci::SecureFile exposes its fields in, and gave each example a value consistent with the name already in it (jks and cer).

I found this while adding the same field to the Go API client, in gitlab-org/api/client-go!3045 (merged), where PatrickRice asked whether I would update the documentation too.

We can see the API returns it here: https://gitlab.com/gitlab-org/gitlab/-/blob/v19.3.1-ee/lib/api/entities/ci/secure_file.rb#L15

MR acceptance checklist

This change is documentation only. The examples are valid JSON and no endpoint, parameter or behaviour changes.