I have seen that in the "Getting Started" section of the wiki, there is a video on how to dual boot, but no steps to just install Windows 11 on the (as recommended) internal storage.
I have done a fork and have written the steps in a basic way, I don't know if you accept Pull Request or how can I push from a wiki that is not in the main repository.
Take a look at it if that's ok with you.
Link to the updated page: https://github.com/rcerrejon/Steamdeck-Ultimate-Windows11-Guide/wiki/1.0-Getting-Started#how-to-install-windows-11-in-your-internal-storage