docs: add Vision & Image Paste guide with platform compatibility

New docs page covering clipboard image paste across all platforms:
- Platform compatibility table (macOS, Linux X11/Wayland, WSL2, VSCode, SSH)
- Setup instructions per platform (xclip, wl-paste, powershell.exe)
- Explanation of terminal paste limitations and why /paste exists
- SSH workarounds (file upload, URLs, X11 forwarding, messaging)
- Keybinding reference (Alt+V, Ctrl+V, /paste) with when each works

Also updates CLI commands reference with /paste command and
Alt+V keybinding documentation.

website/sidebars.ts

@@ -49,6 +49,7 @@ const sidebars: SidebarsConfig = {
             'user-guide/features/code-execution',
             'user-guide/features/browser',
             'user-guide/features/image-generation',
+            'user-guide/features/vision',
             'user-guide/features/tts',
             'user-guide/features/provider-routing',
             'user-guide/features/honcho',