Upstream moved the documentation sources from doc/ to docs/ (#7584), so the
page this branch adds now belongs under docs/doxygen/pages/user_documentation/
gameplay/. The index.md edit auto-merged onto the moved file, but the new page
stayed at the old path, and GitHub's merge engine reported a conflict in
index.md because the file it edits was deleted upstream.
No source changes: the merge is upstream/master as-is plus the doc relocation.
