Add storybook docs addon

Renaming the story files is necessary to conform with internal regex for
selecting files from which the story is extracted[1][2].

[1]: storybookjs/storybook#8104 (comment)
[2]: https://github.com/storybookjs/storybook/blob/next/addons/docs/docs/docspage.md#story-file-names

Overwriting `extractComponentDescription` seems to be the only way to
add back a description into the stories file itself that does not live
in the component.

Change-Id: Ia50d560ad5215e089db938dcfc060f740419118a

.storybook/main.js

@@ -2,5 +2,6 @@ module.exports = {
 	stories: [ '../stories/*.js' ],
 	addons: [
 		'@storybook/addon-a11y',
+		'@storybook/addon-docs',
 	],
 };

.storybook/preview.js

@@ -1,5 +1,11 @@
-import { addDecorator } from '@storybook/vue';
+import { addDecorator, addParameters } from '@storybook/vue';
 import { withA11y } from '@storybook/addon-a11y';
 import './storybook-global.scss';
 
 addDecorator( withA11y );
+
+addParameters( {
+	docs: {
+		inlineStories: true,
+	},
+} );