docs: mention future alternative install types

docs/install/index.md

@@ -1,3 +1,10 @@
 # Installation
 
-You can use either [Docker](docker) (recommended) or your [local Node.js](local) installation.
+You can use either [Docker](docker) (recommended) or your [local Node.js](local) installation.
+
+## Alternatives
+
+These are not officially supported.
+
+- Nix flake (soon)
+- Pterodactyl Egg (soon)