docs: add roadmap to repo (#7107)

docs/tendermint-core/README.md

@@ -1,7 +1,7 @@
 ---
 order: 1
 parent:
-  title: System
+  title: Understanding Tendermint
   order: 5
 ---
 
@@ -10,7 +10,6 @@ parent:
 This section dives into the internals of Go-Tendermint.
 
 - [Using Tendermint](./using-tendermint.md)
-- [Running in Production](./running-in-production.md)
 - [Subscribing to events](./subscription.md)
 - [Block Structure](./block-structure.md)
 - [RPC](./rpc.md)
@@ -18,3 +17,5 @@ This section dives into the internals of Go-Tendermint.
 - [State Sync](./state-sync.md)
 - [Mempool](./mempool.md)
 - [Light Client](./light-client.md)
+
+For full specifications refer to the [spec repo](https://github.com/tendermint/spec).