Add an empty nav.adoc file to make Antora happy
diff --git a/docs/modules/ROOT/nav.adoc b/docs/modules/ROOT/nav.adoc
new file mode 100644
index 0000000..988af44
--- /dev/null
+++ b/docs/modules/ROOT/nav.adoc
@@ -0,0 +1 @@
+// empty, because the pages are just dummy holders of metadata