Make docs clearer regarding local vs public testnets (#823)

* Clear docs regarding local vs public testnets

* Rename private to local

book/src/SUMMARY.md

@@ -7,8 +7,8 @@
 * [Installation](./installation.md)
     * [Docker](./docker.md)
 * [CLI](./cli.md)
-    * [Testnets](./testnets.md)
 	* [Simple Local Testnet](./simple-testnet.md)
+	* [Local Testnets](./local-testnets.md)
 * [API](./api.md)
 	* [HTTP (RESTful JSON)](./http.md)
 	    * [/beacon](./http_beacon.md)

book/src/local-testnets.md

@@ -1,4 +1,7 @@
-# Testnets
+# Local Testnets
+
+> This section is about running your own private local testnets.
+> - If you wish to join the ongoing public testnet, please read [become a validator](./become-a-validator.md).
 
 The `beacon_node` and `validator` commands have a `testnet` sub-command to
 allow creating or connecting to Eth2 beacon chain testnets.

book/src/simple-testnet.md

@@ -1,5 +1,10 @@
 # Simple Local Testnet
 
+> This guide is about running your own private local testnet.
+> - If you wish to join the ongoing public testnet, please read [become a validator](./become-a-validator.md).
+
+This guide will help you setup your own private local testnet.
+
 First, [install Lighthouse](./installation.md).
 
 Then, get the current unix time in seconds; you can use