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/setup.html

@@ -111,7 +111,7 @@
       </div>
     </div>
 
-    <!-- Step 4: Complete (Future: Steam Cloud) -->
+    <!-- Step 4: Complete -->
     <div class="setup-step" data-step="4">
       <div class="step-content">
         <div class="success-icon">✓</div>
@@ -120,13 +120,8 @@
         <div class="completion-summary" id="completion-summary">
           <!-- Summary will be populated dynamically -->
         </div>
-        <div class="future-feature-teaser">
-          <h3>Coming Soon: Steam Cloud Sync</h3>
-          <p class="teaser-text">
-            <span class="teaser-icon">☁️</span>
-            In Phase 2, you'll be able to sync your bookmarks, settings, and themes across devices using Steam Cloud.
-          </p>
-        </div>
+        
+       
       </div>
       <div class="step-actions">
         <button class="btn btn-primary btn-large" id="btn-finish">Start Browsing</button>