pnpm requires direct dependency for fonts; fix #31

Author: mbostock

docs/kit.html

@@ -26,6 +26,12 @@
     npm install @observablehq/notebook-kit
     ```
 
+    If you use [pnpm](https://pnpm.io/), you must also explicitly install any fonts used by the [default styles](https://github.com/observablehq/notebook-kit/blob/main/src/templates/default.html) (unless you plan on using different fonts):
+
+    ```sh
+    pnpm install @observablehq/notebook-kit @fontsource/inter @fontsource/source-serif-4 @fontsource/spline-sans-mono
+    ```
+
     Notebook Kit requires [Node.js](https://nodejs.org/en) 20.19+ or 22.12+.
   </script>
   <script id="47" type="text/markdown">