case 4690 - build docs: pin MkDocs below 2 (Material is incompatible with 2.0); Zensical move on the roadmap

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

devdocs/modernization/ROADMAP.md

@@ -37,6 +37,13 @@ In this order, each step tested before the next:
 5. Webpack → Vite
 6. Optional: Options API → Composition API; TypeScript for new files only
 
+## Step F, customer manual build (not part of the current port)
+
+MkDocs 1.x is unmaintained, and Material for MkDocs does not work with MkDocs 2.0 (the
+build prints a warning to that effect since mkdocs-material 9.7). MkDocs is pinned below 2
+(`devdocs/build/BUILD.md`). Later: move the manual and customer docs to Zensical, the
+Material team's successor, and check every page and link after the move.
+
 ## Before step D starts
 
 1. The integration suite passes, apart from the three known server bugs