Add Subgraphs to docs sidebar

(cherry picked from commit 5e76b2622546a2e42e5c19e4ce1a96ee2691c7fe)

docs/modules/ROOT/nav.adoc

@@ -19,3 +19,5 @@
 * xref:crosschain.adoc[Crosschain]
 
 * xref:utilities.adoc[Utilities]
+
+* xref:subgraphs::index.adoc[Subgraphs]