Add highlevel documentation of content management

Documentation for content management was provided, but needs more work.

Testing locally:

1. Install Drupal CMS locally following the instructions in the README.md of the project
2. Require demo content recipe: ddev composer require drupal/drupal_cms_demo_content
3. Re-install with demo content: ddev drush si ../recipes/drupal_cms_demo_content

Tesing using Drupal CMS online demo:

1. Go to https://www.1xinternet.de/en/drupal-cms-start-demo
2. Request demo with demo content (default option)