[refactor,#527][m]: move the nextjs app at root of repo to examples/dataset-frictionless.

The current app at root of repo is for single frictionless dataset. Should be in its own example so we have space going forward for multiple example and for root to have core portal.js code.

* Also refactor its README (moved from root) to reflect it is just an example
* Move design content from the root README.md into DESIGN.md
* Stub a new root README.md based largely on examples/catalog/README.md

examples/dataset-frictionless/README.md

@@ -0,0 +1,24 @@
+This examples if for presenting a single dataset. The dataset should be a [Frictionless dataset (data package)][fd] i.e. there should be a `datapackage.json`.
+
+[fd]: https://frictionlessdata.io/data-packages/
+
+## Install
+
+Git clone this directory then:
+
+```
+yarn install
+```
+
+## Usage
+
+In this directory:
+
+```bash
+export PORTAL_DATASET_PATH=/path/to/my/dataset
+yarn dev
+```
+
+And you will get a nice dataset page at `http://localhost:3000`
+
+![](https://i.imgur.com/KSEtNF1.png)