📚 Add submission form link to deployment page

The plugin submission page is hard to find while looking at the plugin help docs (As it's not linked from there). It should eventually be a page of its own but there isn't enough content yet (or an illustration) to support it.

docs/plugins/deployment.md

@@ -216,3 +216,9 @@ Success! - Published to example-plugin-penpot.surge.sh
 ```
 
 5. Done!
+
+## 3.5. Submitting to Penpot
+
+To make your finished plugin available in our catalog, submit in on the [plugin submission page](https://penpot.app/penpothub/plugins/create-plugin). Once it becomes available any Penpot user will be able to install and use it.
+
+