Improve documentation on DAG models

[mschauer]

Improve documentation on DAG models, see https://r-causal.github.io/ggdag/articles/intro-to-dags.html for a nice example.

[mschauer]

Let's split this up in parts!

[limitless-100]

@mschauer is this issue still active for 2023 Hacktoberfest?

[mschauer]

Yeah, let me register again. Done.

[mschauer]

This is the important bit:

Any pull/merge request with the hacktoberfest-accepted label, submitted to any public GitHub/GitLab repository, with or without the hacktoberfest topic, will be considered valid for Hacktoberfest.

So you can contribute to this package within Hacktoberfest, happy coding!