Migrate netlify-cms-www site into this repo (#860)

* Add frontmatter to docs files (prep to move)

* Move docs into position for website migration

* Migrate website from netlify-cms-www

Some modifications, including most of the changes in https://github.com/netlify/netlify-cms-www/pull/58 (previously reverted).

Also updated the readme and added hugo-bin for quicker onboarding of new docs contributors.

* Remove netlify.toml

This allows separate build commands for cms-demo and netlifycms.org.

* Remove website/netlify.toml

May re-add later, but it's not doing anything for now.

* Remove unused content file

website/site/content/docs/extending.md

@@ -0,0 +1,117 @@
+---
+title: Extending Widgets
+position: 60
+---
+
+# Extending With Widgets
+
+The NetlifyCMS exposes an `window.CMS` global object that you can use to register custom widgets, previews, and editor plugins. The available widget extension methods are:
+
+* **registerWidget:** lets you register a custom widget.
+* **registerEditorComponent:** lets you add a block component to the Markdown editor.
+
+### Writing React Components inline
+
+The `registerWidget` requires you to provide a React component. If you have a build process in place for your project, it is possible to integrate with this build process.
+
+However, although possible, it may be cumbersome or even impractical to add a React build phase. For this reason, NetlifyCMS exposes two constructs globally to allow you to create components inline: ‘createClass’ and ‘h’ (alias for React.createElement).
+
+## `registerWidget`
+
+Register a custom widget.
+
+```js
+CMS.registerWidget(name, control, \[preview\])
+```
+
+**Params:**
+
+Param | Type | Description
+--- | --- | ---
+`name` | string | Widget name, allows this widget to be used via the field `widget` property in config
+`control` | React.Component \| string | <ul><li>React component that renders the control, receives the following props: <ul><li>**value:** Current field value</li><li>**onChange:** Callback function to update the field value</li></ul></li><li>Name of a registered widget whose control should be used (includes built in widgets).</li></ul>
+[`preview`] | React.Component, optional | Renders the widget preview, receives the following props: <ul><li>**value:** Current preview value</li><li>**field:** Immutable map of current field configuration</li><li>**metadata:** Immutable map of any available metadata for the current field</li><li>**getAsset:** Function for retrieving an asset url for image/file fields</li><li>**entry:** Immutable Map of all entry data</li><li>**fieldsMetaData:** Immutable map of metadata from all fields.</li></ul>
+* **field:** The field type that this widget will be used for.
+* **control:** A React component that renders the editing interface for this field. Two props will be passed:
+  * **value:** The current value for this field.
+  * **onChange:** Callback function to update the field value.
+* **preview (optional):** A React component that renders the preview of how the content will look. A `value` prop will be passed to this component.
+
+**Example:**
+
+```html
+<script src="https://unpkg.com/netlify-cms@^0.7.0/dist/cms.js"></script>
+<script>
+var CategoriesControl = createClass({
+  handleChange: function(e) {
+    this.props.onChange(e.target.value.split(',').map((e) => e.trim()));
+  },
+
+  render: function() {
+    var value = this.props.value;
+    return h('input', { type: 'text', value: value ? value.join(', ') : '', onChange: this.handleChange });
+  }
+});
+
+var CategoriesPreview = createClass({
+  render: function() {
+    return h('ul', {},
+      this.props.value.map(function(val, index) {
+        return h('li', {key: index}, val);
+      })
+    );
+  }
+});
+
+CMS.registerWidget('categories', CategoriesControl, CategoriesPreview);
+</script>
+```
+
+## `registerEditorComponent`
+
+Register a block level component for the Markdown editor:
+
+    CMS.registerEditorComponent(definition)
+
+**Params**
+
+* **definition:** The component definition; must specify: id, label, fields, patterns, fromBlock, toBlock, toPreview
+
+**Example:**
+
+```html
+<script src="https://unpkg.com/netlify-cms@^0.7.0/dist/cms.js"></script>
+<script>
+CMS.registerEditorComponent({
+  // Internal id of the component
+  id: "youtube",
+  // Visible label
+  label: "Youtube",
+  // Fields the user need to fill out when adding an instance of the component
+  fields: [{name: 'id', label: 'Youtube Video ID', widget: 'string'}],
+  // Pattern to identify a block as being an instance of this component
+  pattern: /youtube (\S+)\s/,
+  // Function to extract data elements from the regexp match
+  fromBlock: function(match) {
+    return {
+      id: match[1]
+    };
+  },
+  // Function to create a text block from an instance of this component
+  toBlock: function(obj) {
+    return 'youtube ' + obj.id;
+  },
+  // Preview output for this component. Can either be a string or a React component
+  // (component gives better render performance)
+  toPreview: function(obj) {
+    return (
+      '<img src="http://img.youtube.com/vi/' + obj.id + '/maxresdefault.jpg" alt="Youtube Video"/>'
+    );
+  }
+});
+</script>
+```
+
+**Result:**
+
+![youtube-widget](/img/youtube-widget.png)