Add quickstart guide

Reviewed-by: Aleix Boné <abonerib@bsc.es>

web/content/doc/_index.md

@@ -0,0 +1,10 @@
+---
+title: "Docs"
+description: "Documentation for users of jungle machines"
+date: 2023-09-15
+---
+
+If this is the first time you use any of the jungle machines with NixOS, follow
+the [quick start guide](quickstart).
+
+