[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

.github/workflows/examples-dataset-frictionless.yml

@@ -1,4 +1,4 @@
-name: portal
+name: dataset-frictionless
 on: 
   push:
     branches: [ main ]
@@ -11,7 +11,11 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - name: Install modules
-        run: yarn
+        run: |
+          cd examples/dataset-frictionless
+          yarn
     
       - name: Run tests
-        run: yarn test
+        run: |
+          cd examples/dataset-frictionless
+          yarn test -u