docs: move developing UI to developing Boundary section (#467)

website/content/docs/developing/index.mdx

@@ -1,10 +1,11 @@
 ---
 layout: docs
-page_title: Documentation
+page_title: Developing Boundary
 sidebar_title: Developing Boundary
 description: Developing Boundary
 ---
 
 # Developing Boundary
 
-Please consult our [project on GitHub](https://github.com/hashicorp/boundary) for more information about contributing and developing Boundary.
+The side-bar contains some common development tasks, but for the most up-to-date information about developing Boundary, please consult
+our [project on GitHub](https://github.com/hashicorp/boundary) for more information.

website/content/docs/developing/ui.mdx

@@ -0,0 +1,22 @@
+---
+layout: docs
+page_title: Developing the UI
+sidebar_title: Developing the UI
+description: Developing the Boundary User Interface
+---
+
+# Developing the Boundary User Interface
+
+For detailed instructions and the most up-to-date information on developing the
+Boundary admin UI, please consult [the project README on GitHub](https://github.com/hashicorp/boundary-ui).
+
+## Run a Local Fork of the UI Locally in Dev Mode
+
+If you're developing the Boundary UI locally you can point a dev mode server at a
+local checkout of the repo without building it into the binary. We call this
+directory a passthrough directory, and you can enable it using an env var while in
+conjunction with running Boundary in dev mode:
+
+```bash
+BOUNDARY_DEV_PASSTHROUGH_DIRECTORY=../boundary-ui/ui/core/dist boundary dev
+```