Reorg the markdown book to cater to app devs

First, talk about how a client interacts with Solana to do useful
things. Then describe how the fullnode you're talking to works and
why it's so very fast.  Last, why that fullnode you don't trust
does what you asked it to anyway.

src/SUMMARY.md

@@ -4,10 +4,8 @@
 
 - [Terminology](terminology.md)
 
-- [Synchronization](synchronization.md)
-  - [Introduction to VDFs](vdf.md)
-  - [Proof of History](poh.md)
-  - [Leader Rotation](leader-scheduler.md)
+- [On-chain programs](programs.md)
+  - [The Runtime](runtime.md)
 
 - [Fullnode](fullnode.md)
   - [Tpu](tpu.md)
@@ -15,12 +13,13 @@
   - [Ncp](ncp.md)
   - [JsonRpcService](jsonrpc-service.md)
 
-- [Avalanche replication](avalanche.md)
-  - [Storage](storage.md)
-
-- [On-chain programs](programs.md)
-  - [The Runtime](runtime.md)
-  - [Ledger format](ledger.md)
+- [A Solana Cluster](cluster.md)
+  - [Synchronization](synchronization.md)
+    - [Introduction to VDFs](vdf.md)
+    - [Proof of History](poh.md)
+  - [Ledger Broadcasting](avalanche.md)
+  - [Ledger Replication](storage.md)
+  - [Leader Rotation](leader-scheduler.md)
 
 ## Appendix
 

src/avalanche.md

@@ -1,4 +1,4 @@
-# Avalanche replication
+# Ledger Broadcasting
 
 The [Avalance explainer video](https://www.youtube.com/watch?v=qt_gDRXHrHQ) is
 a conceptual overview of how a Solana leader can continuously process a gigabit

src/cluster.md

@@ -0,0 +1,10 @@
+# A Solana Cluster
+
+A Solana cluster is a set of fullnodes working together to serve client
+transactions and maintain the integrity of the ledger. Many clusters may
+coexist.  When two clusters share a common genesis block, they attempt to
+converge. Otherwise, they simply ignore the existence of the other.
+Transactions sent to the wrong one are quietly rejected. In this chapter,
+we'll discuss how a cluster is created, how nodes join the cluster, how
+they share the ledger, how they ensure the ledger is replicated, and how
+the cope with buggy and malicious nodes.

src/ledger.md

@@ -1 +0,0 @@
-# Ledger format

src/storage.md

@@ -1,4 +1,4 @@
-# Storage
+# Ledger Replication
 
 ## Background