explain a bit about about zrok frontdoor in the install guide

docs/guides/frontdoor.mdx

@@ -12,14 +12,20 @@ import TabItem from '@theme/TabItem';
 import LinuxService from './_frontdoor-linux.mdx';
 import ReservedDocker from './_frontdoor-docker.mdx';
 
-**zrok frontdoor** provides a shielded entry point for your production website or service. This is useful if you want to expose it to the public internet, but not directly.
-
-## Video Tour
+**zrok frontdoor** is the heavy-duty front door to your app or site. It makes your website or app available to your online audience through the shield of zrok.io's hardened, managed frontends.
 
 <iframe width="100%" height="315" src="https://www.youtube.com/embed/5Vi8GKuTi_I" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe>
 
+  ## Overview
+
+zrok frontends are the parts of zrok that proxy incoming public web traffic to zrok backend shares via OpenZiti. When you use zrok with a `zrok.io` frontend, you're using **zrok frontdoor**. `zrok.io` is zrok-as-a-service by NetFoundry, the team behind OpenZiti. You need a free account to use **zrok frontdoor**.
+
+![diagram](/img/zrok-frontdoor.excalidraw.png)
+
 ## Choose your OS
 
+Choose between installing the Linux package or running zrok with Docker (Linux, macOS, or Windows).
+
 <OsTabs
   queryString="os"
   values={[