docs: add blurb in dev mode doc about UI passthrough (#362)

website/pages/docs/installing/dev-mode.mdx

@@ -66,3 +66,16 @@ An example using these values to configure your Terraform provider instance for
 ```
 
 Note in the example above we're setting `base_url` to `http` and not `https` as the Boundary server does not use TLS in development mode.
+
+## User Interface
+
+The UI is normally compiled into the Boundary binary at build time.
+However, it would be impractical to rebuild the binary on every change when
+actively developing the UI.  To make UI development more convenient, the binary
+supports a _passthrough directory_.  This is an arbitrary local
+directory from which UI assets are served.  Note this option is only available
+in dev mode.  For example:
+
+```bash
+BOUNDARY_DEV_PASSTHROUGH_DIRECTORY=../boundary-ui/ui/core/dist ./boundary dev
+```