docs: fix broken dashboard image on i18n pages (#38031)

The dashboard screenshot uses a relative path `src="whatsapp-openclaw.jpg"`
which resolves correctly on the English root page but produces 404 on
zh-CN and ja-JP pages because Mintlify prepends the language subdirectory
to the CDN path.

Change to absolute path `/whatsapp-openclaw.jpg` in all three index files,
consistent with other images on the same page that already use absolute
paths (e.g. `/assets/openclaw-logo-text-dark.png`).

docs/index.md

@@ -124,7 +124,7 @@ Open the browser Control UI after the Gateway starts.
 - Remote access: [Web surfaces](/web) and [Tailscale](/gateway/tailscale)
 
 <p align="center">
-  <img src="whatsapp-openclaw.jpg" alt="OpenClaw" width="420" />
+  <img src="/whatsapp-openclaw.jpg" alt="OpenClaw" width="420" />
 </p>
 
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