Add link to summary column (#2135)

Author: Yuji Sugiura

guide/src/SUMMARY.md

@@ -22,6 +22,7 @@
   - [web-sys: WebAudio](./examples/web-audio.md)
   - [web-sys: WebGL](./examples/webgl.md)
   - [web-sys: WebSockets](./examples/websockets.md)
+  - [web-sys: WebRTC DataChannel](./examples/webrtc_datachannel.md)
   - [web-sys: `requestAnimationFrame`](./examples/request-animation-frame.md)
   - [web-sys: A Simple Paint Program](./examples/paint.md)
   - [Parallel Raytracing](./examples/raytrace.md)