Reorganize docs, add codepen examples (rjsf-team#1691)

### Reasons for making this change

Reorganize docs to make them easier for beginners to use, as well as giving them a more comprehensive API reference for the various props / widgets / fields that rjsf supports. (see rjsf-team#1692)


![image](https://user-images.githubusercontent.com/1689183/81511033-69982980-92e4-11ea-96f9-7cc94386b556.png)

Also add codepen examples (see `docs/main.js` for the code that converts all eligible ```jsx blocks to codepen examples):

![image](https://user-images.githubusercontent.com/1689183/81511036-7026a100-92e4-11ea-8d6e-0e6d09b36a43.png)

Also upgraded mkdocs to 1.1.

.github/workflows/ci.yml

@@ -62,5 +62,5 @@ jobs:
       uses: actions/setup-python@v1.2.0
       with:
         python-version: 3.6
-    - run: pip install mkdocs==1.0.4
+    - run: pip install -r requirements.docs.txt
     - run: mkdocs build --clean

README.md

@@ -1,14 +1,14 @@
 react-jsonschema-form
 =====================
 
-[![Build Status](https://travis-ci.org/mozilla-services/react-jsonschema-form.svg?branch=master)](https://travis-ci.org/mozilla-services/react-jsonschema-form)
+![Build Status](https://github.com/rjsf-team/react-jsonschema-form/workflows/CI/badge.svg)
 
 A simple [React](http://facebook.github.io/react/) component capable of building HTML forms out of a [JSON schema](http://json-schema.org/) and using [Bootstrap](http://getbootstrap.com/) semantics by default.
 
 ![](http://i.imgur.com/M8ZCES5.gif)
 
 Testing powered by BrowserStack<br>
-<a target=_blank href="https://www.browserstack.com/"><img width=200 src="https://user-images.githubusercontent.com/1689183/51487090-4ea04f80-1d57-11e9-9a91-79b7ef8d2013.png"></a>
+<a target=_blank href="https://www.browserstack.com/"><img width="200" src="https://user-images.githubusercontent.com/1689183/51487090-4ea04f80-1d57-11e9-9a91-79b7ef8d2013.png"></a>
 
 
 ## Documentation