docs: Add steam deck info from README

docs/devices/valve-steam-deck/index.md

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+Steam Deck
+==========
+
+Configuration
+-------------
+
+When configuring the system, import `./modules` from this repo in your configuration and enable the Steam Deck-specific hardware configurations with `jovian.devices.steamdeck.enable = true;`.
+
+
+Firmware Updates
+----------------
+
+Firmware update, at this time, is not handled automatically.
+
+### `steamdeck-firmware`
+
+Updates to the BIOS and the controller are available in the `steamdeck-firmware` package:
+
+- BIOS: Run `sudo jupiter-biosupdate`
+- Controller: Run `sudo jupiter-controller-update`
+
+### `jupiter-dock-updater-bin`
+
+Updates to the Docking Station firmware are available in the `jupiter-dock-updater-bin` package.
+Connect to the dock via USB-C and run `jupiter-dock-updater` to update.
+
+
+Helpful Tips
+------------
+
+To enter the boot menu, power off the Steam Deck then hold down `Volume-` and tap the `Power` button.