doc: draw attention to release branches (#726)

Using the wrong branch is a very common cause of questions, due to it
not being mentioned anywhere in the documentation.

docs/src/installation.md

@@ -23,6 +23,16 @@ screens, and display managers.
 ```
 <small>Minimal `flake.nix` for a NixOS configuration.</small>
 
+If you are using a stable release of NixOS, ensure that you use the matching
+Stylix release. For example:
+
+```nix
+{
+  nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.11";
+  stylix.url = "github:danth/stylix/release-24.11";
+}
+```
+
 Many applications cannot be configured system wide, so Stylix will also need
 [Home Manager][nix-hm] to be able to change their settings within your home
 directory.
@@ -94,6 +104,17 @@ is managed by someone else.
 ```
 <small>Minimal `flake.nix` for a Home Manager configuration.</small>
 
+If you are using a stable release of Home Manager, ensure that you use the
+matching Stylix release. For example:
+
+```nix
+{
+  nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.11";
+  home-manager.url = "github:nix-community/home-manager/release-24.11";
+  stylix.url = "github:danth/stylix/release-24.11";
+}
+```
+
 If you choose to use both NixOS and Home Manager but configure them separately,
 you will need to copy the settings described below into both of your
 configurations, as keeping them separate means that they cannot follow each