reduce scope of nixos documentation

There are so many ways to do this we realistically shouldn't bother
describing any of them, especially because people should be learning all
the options and choosing the one that suits them best anyway.

docs/deploying/nixos.md

@@ -0,0 +1,18 @@
+# Conduit for NixOS
+
+Conduit can be acquired by Nix from various places:
+
+* The `flake.nix` at the root of the repo
+* The `default.nix` at the root of the repo
+* From Nixpkgs
+
+The `flake.nix` and `default.nix` do not (currently) provide a NixOS module, so
+(for now) [`services.matrix-conduit`][module] from Nixpkgs should be used to
+configure Conduit.
+
+If you want to run the latest code, you should get Conduit from the `flake.nix`
+or `default.nix` and set [`services.matrix-conduit.package`][package]
+appropriately.
+
+[module]: https://search.nixos.org/options?channel=unstable&query=services.matrix-conduit
+[package]: https://search.nixos.org/options?channel=unstable&query=services.matrix-conduit.package