usage: getting-started: cover how to install and run on steam deck

Start at the getting-started documentation entry point in Cockpit-docs and compare its current installation guidance with the Steam Deck steps listed here. Add the supported Desktop and Gaming Mode workflow, release download and installation details, launch guidance, and current update limitations; done means a Steam Deck user can follow the documented steps successfully.

From previous discussion with @patrickelectric in August:

1. Click and hold the power button to Switch to Desktop mode
2. Bottom left button (Home) / Internet / open a browser (may need to install one first - Firefox recommended)
3. Download the .flatpak from the Assets section of the latest/relevant GitHub release
   • Find it at https://github.com/bluerobotics/cockpit/releases (for less typing, search for “bluerobotics cockpit”, click the GitHub page, then Tags, then Releases)
   • We’re hoping to make this easier, either
     • through the Flatpak “Discover” store, and/or
     • through a short URL that directly installs and/or configures it for you
4. Install the flatpak once it’s downloaded
   • From the Downloads section of the file browser
5. Bottom left button / All Applications → find Cockpit and right click to "Add to Steam"
   • If you can't right click then
     1. Click the Steam icon on the right side of the bottom bar
     2. Click “Store”
     3. Click Games (top menu-bar)
     4. Add a non steam game to library
     5. Find and add Cockpit
6. Click "Return to Gaming Mode" on the desktop
7. Launch Cockpit from "Non-Steam" section of your library
   • For testing purposes, can also be launched in desktop mode from "All Applications", or the Steam library, but to use the joystick you’ll need to enable gamepad mode by holding the burger menu joystick button for a few seconds, and full screen use may be awkward
   • Before launching, you can also adjust the application properties by clicking the cog icon, e.g.
     • Can change resolution
     • Can change Controller Settings to do things like
       • set a custom joystick mapping
         • We’re hoping to improve Cockpit’s internal interface for Steam Deck joystick support, potentially by including a default configuration for it in the Steam per-application controller database
       • set Gyro to “Joystick”, to allow tilting and rolling the device to provide control axis inputs
8. Updating currently has some issues
   • We’re working on a fix to properly support version numbers, so installing a new version overwrites the old one by default
   • We’re also working on a way to update automatically / from within the application, instead of needing to go through the whole installation process again