docs: add abci spec to config.js

docs/config.js

@@ -63,6 +63,15 @@ module.exports = {
           "/app-dev/ecosystem"
         ]
       },
+      {
+        title: "ABCI Specification",
+        collapsable: false,
+        children: [
+          "/spec/abci/abci",
+          "/spec/abci/apps",
+          "/spec/abci/client-server"
+        ]
+      },
       {
         title: "Research",
         collapsable: false,