Introduce archetypes

[StefMa]

https://gohugo.io/content-management/archetypes/

The only single.html type page in the theme is the the exampleSite/content/agb.md which (I believe) is included to show examples of using the shortcodes. In it's front matter is the "include_footer" page parameter. I would like to suggest that this theme could make use of at least a couple archetype files (to be included in the exampleSite). One could be used to create pages like the agb.md file (showing how the "include_footer" page parameter can automatically be put in the pages found in the root of the content folder (saving a little typing). And I think it would be good to show a nested archetype file for the posts made in content/blog/posts/

See also Point 5 from #135