doc: add documentation for testbeds (#347)

Co-authored-by: NAHO <90870942+trueNAHO@users.noreply.github.com>

docs/src/SUMMARY.md

@@ -15,4 +15,5 @@
 
 - [Contributing](contributing.md)
 - [Adding modules](modules.md)
+- [Testbeds](testbeds.md)
 - [Style guide](styling.md)

docs/src/testbeds.md

@@ -0,0 +1,83 @@
+# Testbeds
+
+Stylix provides a suite of virtual machines which can be used to test and
+preview themes without installing the target to your live system.
+
+These can be particularly helpful for:
+
+- Working on targets before the login screen, since you can avoid closing
+  your editor to see the result.
+- Developing for a different desktop environment than the one you normally use.
+- Reducing the risk of breaking your system while reviewing pull requests.
+
+Testbeds are also built by GitHub Actions for every pull request. This is less
+beneficial compared to running them yourself, since it cannot visually check
+the theme, however it can catch build failures which may have been missed
+otherwise.
+
+## Creation
+
+New testbeds are defined by creating a file called `testbed.nix` within the
+folder for the corresponding target. This file will automatically be loaded
+as a NixOS module, with options such as `stylix.image` already defined.
+The module should include any options necessary to install the target and
+any supporting software - for example, a window manager.
+
+If the target can only be used through Home Manager, you can write a
+Home Manager module within the NixOS module using the following format:
+
+```nix
+{
+    home-manager.sharedModules = [{
+        # Write Home Manager options here
+    }];
+}
+```
+
+Using `home-manager.sharedModules` is preferred over `home-manager.users.guest`
+since it allows us to easily change the username or add additional users in
+the future.
+
+Once the module is complete, use `git add` to track the file, then the new
+packages will be [available to use](#usage).
+
+## Usage
+
+You can list the available testbeds by running this command from anywhere
+within the repository:
+
+```console
+user@host:~$ nix flake show
+github:danth/stylix
+└───packages
+    └───x86_64-linux
+        ├───docs: package 'stylix-book'
+        ├───palette-generator: package 'palette-generator'
+        ├───testbed-gnome-dark: package 'testbed-gnome-dark'
+        ├───testbed-gnome-light: package 'testbed-gnome-light'
+        ├───testbed-kde-dark: package 'testbed-kde-dark'
+        └───testbed-kde-light: package 'testbed-kde-light'
+```
+
+(This has been edited down to only the relevant parts.)
+
+To start a testbed, each of which is named in the format
+`testbed-«target»-«polarity»`, run the following command:
+
+```console
+user@host:~$ nix run .#testbed-«target»-«polarity»
+```
+
+Any package with a name not fitting the given format is not a testbed,
+and may behave differently with this command, or not work at all.
+
+Once the virtual machine starts, a window should open, similar to the screenshot
+below. The contents of the virtual machine will vary depending on the target you
+selected earlier.
+
+![GDM login screen with a dark background color and showing a guest user](testbed-gnome-dark.png)
+
+If the testbed includes a login screen, the guest user should log in
+automatically when selected. Depending on the software used, you may still be
+presented with a password prompt - in which case you can leave it blank and
+proceed by pressing enter.