docs: Restructure table of contents (#3230)

* docs: Restructure table of contents

* docs: Copy edits to Develop/Get started sections

* docs: Add index to HCP Boundary section

* docs: Move OSS/Install section to top-level

* docs: Move Concepts above Configuration

website/content/docs/developing/building.mdx

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+---
+layout: docs
+page_title: Build Boundary
+description: Build Boundary from source
+---
+
+# Build Boundary
+
+Building Boundary from source requires:
+
+- [Go](https://golang.org/dl/). Boundary developers keep current with new
+  releases of Go in order to take advantage of new features and to ensure that
+  security issues within Go are addressed. If building Boundary yourself, always
+  use the latest version and keep current.
+- `make` installed
+- [Docker](https://docs.docker.com/get-docker/) installed
+- Basic development environment (git, xcode if you're on Mac, etc)
+
+1. `$ git clone https://github.com/hashicorp/boundary` or if you're using SSH auth for
+   github, `$ git clone git@github.com:hashicorp/boundary`
+2. `$ cd boundary`
+3. `$ make install`
+
+The first time you run `make install` it will build UI assets, which will take a
+long time. After the initial build, you can update those assets to the
+currently-pinned version via `make build-ui`.
+
+Once you've built Boundary, the binary will be available at `<repo path>/bin/boundary`. Move this
+binary to somewhere in your `$PATH` such as `/usr/local/bin`.
+
+## Cross-platform build
+
+If you need to cross compile boundary for another OS or architecture, use the [GOOS and GOARCH](https://github.com/hashicorp/boundary/blob/master/scripts/build.sh#L26)
+environment variable to set this. Example: `$ GOOS=linux GOARCH=amd64 make install`
+
+For more details, please consult the [Boundary project on GitHub](https://github.com/hashicorp/boundary#build-and-start-boundary-in-dev-mode).
+
+## Troubleshooting
+
+UI assets are built inside a Docker container. If this build step fails, try
+increasing memory and swap available to Docker.

website/content/docs/developing/index.mdx

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+---
+layout: docs
+page_title: Develop Boundary
+description: Develop Boundary
+---
+
+# Develop 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

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+---
+layout: docs
+page_title: Develop the UI
+description: Develop the Boundary user interface
+---
+
+# Develop 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_UI_PASSTHROUGH_DIR=../boundary-ui/ui/admin/dist boundary dev
+```