docs: Init configuration with some info from README

docs/configuration.md

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+Configuration
+=============
+
+> [!WARNING]
+> This page is a stub.
+
+All available module options along with their descriptions can be found under `modules`.
+
+To use the Steam Deck UI, set `jovian.steam.enable = true;` in your configuration.
+This will only enable the Steam Deck UI tooling. To enable "desktop" steam, enable the usual NixOS options.
+Then you can start the UI using one of the following methods:
+
+- (**preferred**) Use `jovian.steam.autoStart = true;` to auto-start at login, and enabling use of *Switch to Desktop* option.
+- Select "Gaming Mode" in the Display Manager or run `steam-session` in a VT.
+- Launch `steam-session` within an existing desktop session. This will run [gamescope](https://github.com/Plagman/gamescope) in nested mode which results in higher latency.
+
+If you want *Switch to Desktop* to switch to another session with `autoStart`, you will need to configure `jovian.steam.desktopSession`.
+Configure it with the name of the X11 or wayland session of your choosing.
+The *Switch to Desktop* option will not work with the other two methods of running the Steam Deck UI, instead if will close Steam.
+