refactor(i18n): rename docs/plaene/ -> docs/plans/

Last German folder name in demo-epb. Pairs cleanly with docs/plans-md/
(markdown source) following the project convention. All references
in landing page generator, CI workflows, and cross-doc links updated.

tools/generate_landing_page.py

@@ -125,7 +125,7 @@ def main() -> int:
     n_stubs = sum(1 for f in (REPO / "src" / "stubs").glob("*.h"))
 
     # Word docs
-    plans   = collect_docs("docs/plaene")
+    plans   = collect_docs("docs/plans")
     safety  = collect_docs("docs/safety")
     manuals = collect_docs("docs/manuals")
     reviews = collect_docs("docs/reviews")