Update storybook template (#61)

Extend storybook template to include *.mdx files and removed support for
*.stories.mdx files, as with storybook v7 ***.stories.mdx files are
deprecated**. They'll not be supported in a future version anymore:

Quote from [storybook](https://storybook.js.org/docs/writing-docs/mdx):

> If you’re coming from a previous version of Storybook, you might be
accustomed to using MDX both for documentation and for defining stories
in the same .stories.mdx file. We’ve deprecated this functionality and
plan to remove it in a future version of Storybook.

config/storybook.main.template.js

@@ -2,7 +2,7 @@
  * @type {import('@storybook/react-webpack5').StorybookConfig}
  */
 module.exports = {
-  stories: ['../src/**/*.stories.mdx', '../src/**/*.stories.@(js|jsx|ts|tsx)'],
+  stories: ['../src/**/*.mdx', '../src/**/*.stories.@(js|jsx|ts|tsx)'],
   addons: [
     '@storybook/addon-links',
     '@storybook/addon-essentials',