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.js

@@ -416,13 +416,6 @@ function renderCompletionSummary() {
         <div class="summary-value">${setupState.defaultBrowserSet ? 'Set as Default' : 'Not Set'}</div>
       </div>
     </div>
-    <div class="summary-item">
-      <div class="summary-icon">☁️</div>
-      <div class="summary-content">
-        <div class="summary-label">Steam Cloud Sync</div>
-        <div class="summary-value">Coming in Phase 2</div>
-      </div>
-    </div>
   `;
 }