fix: ci: Ensure changelog job builds docs with the new entry

The changelog job is supposed to test that the text from GitLab MR title&description is valid rst syntax and can be built with sphinx. In 49128fc1, the way gitchangelog generates entries was changed - it no longer writes to the changelog file, but generates output on stdout instead. Ensure the generated notes is actually written to (some) rendered file which is part of the docs so that the subsequent sphinx build attempts to render the note.