Update web embedding docs (rerun-io#8359)

### What

* Add a line about what `RRD_URL` can be when used in an iframe

It's the same everywhere, but it's normal to assume that the iframe has
different limitations compared to embedding the web viewer directly.

Also bumped the version, as it was pretty ancient.

docs/content/howto/integrations/embed-web.md

@@ -17,10 +17,12 @@ To implement this, fill in the placeholders:
 - `RRD_URL` - The URL of the recording to display in the viewer.
 - `RERUN_VERSION` - The version of the Rerun SDK used to generate the recording.
 
+The `RRD_URL` can be a file served over `http` (e.g. `https://app.rerun.io/version/0.20.3/examples/arkit_scenes.rrd`), or a connection to an SDK using our [serve](https://www.rerun.io/docs/reference/sdk/operating-modes#serve) API (e.g. `ws://localhost:4321`).
+
 For instance:
 
 ```html
-<iframe src="https://app.rerun.io/version/0.14.1/?url=https://app.rerun.io/version/0.14.1/examples/arkit_scenes.rrd"></iframe>
+<iframe src="https://app.rerun.io/version/0.20.3/?url=https://app.rerun.io/version/0.20.3/examples/arkit_scenes.rrd"></iframe>
 ```
 
 ## Using the JavaScript package
@@ -94,7 +96,7 @@ await viewer.start(null, parentElement);
 The Viewer API supports adding and removing recordings:
 
 ```js,id=embed-web-viewer-api-js-open-close
-const rrdUrl = "https://app.rerun.io/version/0.14.1/examples/arkit_scenes.rrd";
+const rrdUrl = "https://app.rerun.io/version/0.20.3/examples/arkit_scenes.rrd";
 
 // Open a recording:
 viewer.open(rrdUrl);