Hardcode default config, and document how config works

.config/config.json5

@@ -3,15 +3,7 @@
     {
       "user_id": "@alice:example.org",
       "password": "hunter2",
+      // Optional: "device_name": "ratatrix on <hostname>",
     },
   ],
-  "keybindings": {
-    "Home": {
-      "<Ctrl-d>": "/quit",
-      "<Ctrl-c>": "/quit",
-      "<Ctrl-z>": "/suspend",
-      "<Alt-left>": "/previous",
-      "<Alt-right>": "/next",
-    },
-  }
 }