docs: try to make window inline

docs/source/websim.md

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+# Live Caustics Playground
+
+Here is a little online simulator run using caustics! It's slow because it's
+running on a free server, but it's a good way to play with the simulator without
+having to install anything.
+
+Pro tip: check out the "Pixelated" source to lens any image you want!
+
+<div style="display: flex; height: 500px;">
+    <iframe src="https://ciela-institute-caustics-webapp-guistreamlit-app-yanhhm.streamlit.app/?embed=true" style="flex-grow: 1; width: 100%;"></iframe>
+</div>
+
+If the window doesn't show up just follow this link:
+[caustics-webapp](https://ciela-institute-caustics-webapp-guistreamlit-app-yanhhm.streamlit.app)
+
+For frequent simulator users (e.g., if you plan on exploring the parameter space
+of a lens), we recommend installing the simulator locally and running it in your
+browser. Follow the steps below:
+
+1. Install Caustics. Please follow the instructions on the
+   [install page](https://caustics.readthedocs.io/en/latest/install.html).
+2. `pip install streamlit`
+3. `git clone https://github.com/Ciela-Institute/caustics-webapp.git`
+4. Move into the `caustics-webapp/gui/` directory and run the following command:
+   `streamlit run streamlit_app.py --server.enableXsrfProtection=false`
+
+If you were successful in installing the simulator, Step 4 should automatically
+open the simulator in your default browser.