docs: fix broken links and layout (#7154)

This PR does a few minor touch ups to the docs

docs/tendermint-core/README.md

@@ -5,18 +5,20 @@ parent:
   order: 5
 ---
 
+# Overview
+
 This section dives into the internals of Go-Tendermint.
 
 - [Using Tendermint](./using-tendermint.md)
 - [Subscribing to events](./subscription.md)
 - [Block Structure](./block-structure.md)
 - [RPC](./rpc.md)
-- [Block Sync](./block-sync.md)
-- [State Sync](./state-sync.md)
-- [Mempool](./mempool.md)
+- [Block Sync](./block-sync/README.md)
+- [State Sync](./state-sync/README.md)
+- [Mempool](./mempool/README.md)
 - [Light Client](./light-client.md)
 - [Consensus](./consensus/README.md)
-- [Pex](./pex/README.md)
+- [Peer Exachange (PEX)](./pex/README.md)
 - [Evidence](./evidence/README.md)
 
 For full specifications refer to the [spec repo](https://github.com/tendermint/spec).

docs/tendermint-core/consensus/README.md

@@ -5,6 +5,7 @@ parent:
   order: 6
 ---
 
+# Consensus
 
 Tendermint Consensus is a distributed protocol executed by validator processes to agree on
 the next block to be added to the Tendermint blockchain. The protocol proceeds in rounds, where