[ADF-1586] Added new Core doc files (#2383)

docs/accordion-group.component.md

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+# Accordion Group component
+
+Adds a collapsible panel to an [accordion menu](accordion.component.md).
+
+![Accordion menu screenshot](docassets/images/accordion-menu.png)
+
+<!-- markdown-toc start - Don't edit this section.  npm run toc to generate it-->
+
+<!-- toc -->
+
+- [Basic Usage](#basic-usage)
+  * [Properties](#properties)
+- [Details](#details)
+- [See also](#see-also)
+
+<!-- tocstop -->
+
+<!-- markdown-toc end -->
+
+## Basic Usage
+
+```html
+<adf-accordion>
+    <adf-accordion-group [heading]="titleHeading" [isSelected]="true" [headingIcon]="'assignment'">
+        <my-list></my-list>
+    </adf-accordion-group>
+</adf-accordion>
+```
+
+```ts
+@Component({
+    selector: 'my-component'
+})
+export class MyComponent implements OnInit {
+
+    titleHeading: string;
+
+    constructor() {
+        this.titleHeading = 'My Group';
+    }
+
+}
+```
+
+### Properties
+
+| Name | Type | Description |
+| --- | --- | --- |
+| heading | string | Title heading for the group. |
+| isSelected | boolean | Is this group currently selected? |
+| headingIcon | string | The material design icon. |
+| hasAccordionIcon | boolean | Should the (expanded) accordion icon be shown? Defaults to true |
+
+## Details
+
+Place one or more accordion groups within an [Accordion component](accordion.component.md) to define
+a menu. 
+
+<!-- Don't edit the See also section. Edit seeAlsoGraph.json and run config/generateSeeAlso.js -->
+<!-- seealso start -->
+## See also
+
+- [Accordion component](accordion.component.md)
+<!-- seealso end -->