Archive Steam docs; add itch.io upload guide

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.

renderer/settings.html

@@ -36,7 +36,7 @@
         <!-- Big Picture Mode -->
         <div class="setting-group">
           <h3>Big Picture Mode</h3>
-          <p class="note">A controller-friendly UI designed for Steam Deck and handheld devices.</p>
+          <p class="note">A controller-friendly UI designed for handheld devices (e.g., Steam Deck).</p>
           <div class="setting-row">
             <button id="launch-bigpicture-btn" class="primary-btn">
               <span style="font-size: 18px;"></span> Launch Big Picture Mode
@@ -287,7 +287,7 @@
 
         <div class="customization-group">
           <h3>Security Updates</h3>
-          <p class="note" style="margin-bottom: 12px;">Nebula Browser is distributed via Steam, but the Electron runtime can be updated separately for security patches and performance improvements.</p>
+          <p class="note" style="margin-bottom: 12px;">Nebula Browser is distributed via itch.io and other non‑Steam channels; the Electron runtime can be updated separately for security patches and performance improvements.</p>
 
           <div class="stack">
             <!-- Update Status Banner -->