[9.18] chg: doc: Track changes in separate files
Keeping a single changelog file creates annoying conflicts when rebasing the -S version. To eliminate these, keep the changelog for each version in a separate file and also create files for the -S versions to include them in the docs. In the open source version, these files will remain empty and thus won't affect the documentation, while in the -S edition they'll be filled in and included without introducing any conflicts.
Closes #4946 (closed)