Validator on-boarding docs (#656)

* Add first draft of validator onboarding

* Update docs

* Add documentation link to main README

* Continue docs development

* Update book readme

* Update docs

* Allow vc to run without testnet subcommand

* Small change to onboarding docs

* Tidy CLI help messages

* Update docs

* Add check to val client see if beacon node is synced

* Add notifier service to validator client

* Re-order onboarding steps

* Update deposit contract address

* Update testnet dir

* Add note about public eth1 node

* Set default eth1 endpoint to sigp

* Fix broken test

* Try fix eth1 cache locking

* Be more specific about eth1 endpoint

* Increase gas limit for deposit

* Fix default deposit amount

validator_client/src/config.rs

@@ -80,10 +80,13 @@ impl Config {
                             .into(),
                     );
                 }
-                process_testnet_subcommand(sub_cli_args, config)
+                process_testnet_subcommand(sub_cli_args, config)?
             }
-            _ => return Err("You must use the testnet command. See '--help'.".into()),
-        }?;
+            _ => {
+                config.key_source = KeySource::Disk;
+                config
+            }
+        };
 
         Ok(config)
     }