Move legacy Steam-related docs into documentation/archived and add a new UPLOAD-ITCH.md describing how to publish builds to itch.io with Butler. Update top-level README to state official releases will be on itch.io. Remove Steam-specific UI/features: drop steamCloudOptIn from first-run preferences, remove the Steam Cloud teaser and summary from the setup flow, and adjust settings/setup copy to reference handheld devices and non‑Steam distribution. Also make a small wording tweak in the plugins doc about rendererPreload.
