Update Examples page with embeds (#3320)

* Add a package.json to counter-vanilla for CodeSandbox use

* Fix example links and add first sandbox embed

* Try adding Parcel setup for counter-vanilla

* Add counter-vanilla sandbox with temp branch link

* Add sandboxes for other examples

* Add counter-vanilla README

* Format readme

* Remove todos-flow embed due to compile issues

Author: markerikson

counter-vanilla/README.md

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+# Redux Counter Vanilla Example
+
+This example does not require a build system or a view framework, and exists to show the raw Redux API used with ES5.
+
+As Dan said [in the original PR for this example]():
+
+> The new Counter Vanilla example is aimed to dispel the myth that Redux requires Webpack, React, hot reloading, sagas, action creators, constants, Babel, npm, CSS modules, decorators, fluent Latin, an Egghead subscription, a PhD, or an Exceeds Expectations O.W.L. level.
+>
+> Nope, it's just HTML, some artisanal `<script>` tags, and plain old DOM manipulation. Enjoy!
+
+To try this example, just download the HTML file and open it in your browser.
+
+(The `package.json` in this folder only exists to allow using this folder as a CodeSandbox embed, and is not required
+to run the HTML file.)

counter-vanilla/package.json

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+{
+  "name": "counter-vanilla",
+  "version": "1.0.0",
+  "description": "A simple counter implemented using Redux",
+  "main": "index.html",
+  "scripts": {
+    "start": "parcel index.html --open",
+    "build": "parcel build index.html"
+  },
+  "dependencies": {},
+  "devDependencies": {
+    "@babel/core": "^7.2.0",
+    "parcel-bundler": "^1.6.1"
+  }
+}