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/validation.md

@@ -0,0 +1,59 @@
+---
+title: Validation
+position: 70
+---
+
+# Collection Field Validation
+
+## Available validations to use on `config.yml`:
+
+- Presence: By default all widgets are required, unless specified in the config. Example:
+`- {label: "Subtitle", name: "subtitle", widget: "string", required: false}`
+
+- Pattern: Field configuration can specify a regex pattern with the appropriate error message. Example:
+`- {label: "Title", name: "title", widget: "string", pattern: ['.{10,}', "Should have more than 10 characters"] }`
+
+
+## Advanced Guide (For widget authors)
+
+The widget control can optionally implement an `isValid` method to perform custom validations, in addition to presence and pattern. The `isValid` method will be automatically called, and it can return either a boolean value, an object with an error message or a promise. Examples:
+
+**Boolean**
+No errors:
+
+```javascript
+  isValid = () => {
+    // Do internal validation
+    return true;
+  };
+```
+
+Existing error:
+
+```javascript
+  isValid = () => {
+    // Do internal validation
+    return false;
+  };
+```
+
+**Object with `error` (useful for returning custom error messages)**
+Existing error:
+
+```javascript
+  isValid = () => {
+    // Do internal validation
+    return { error: 'Your error message.' };
+  };
+```
+
+**Promise**
+You can also return a promise from `isValid`. While the promise is pending, the widget will be marked as "in error". When the promise resolves, the error is automatically cleared.
+
+```javascript
+  isValid = () => {
+    return this.existingPromise;
+  };
+```
+
+Note: Do not create a promise inside `isValid` - `isValid` is called right before trying to persist. This means that even if a previous promise was already resolved, when the user hits 'save', `isValid` will be called again. If it returns a new promise, it will be immediately marked as "in error" until the new promise resolves.